diff --git a/dlls/mmsystem.dll16/mmio16.c b/dlls/mmsystem.dll16/mmio16.c
index 1db82c2eef2..df28a66357b 100644
--- a/dlls/mmsystem.dll16/mmio16.c
+++ b/dlls/mmsystem.dll16/mmio16.c
@@ -424,10 +424,10 @@ MMRESULT16 WINAPI mmioSetInfo16(HMMIO16 hmmio, const MMIOINFO16* lpmmioinfo, UIN
 /**************************************************************************
  * 				mmioSetBuffer		[MMSYSTEM.1217]
  */
-MMRESULT16 WINAPI mmioSetBuffer16(HMMIO16 hmmio, LPSTR pchBuffer,
+MMRESULT16 WINAPI mmioSetBuffer16(HMMIO16 hmmio, SEGPTR pchBuffer,
                                   LONG cchBuffer, UINT16 uFlags)
 {
-    MMRESULT    ret = mmioSetBuffer(HMMIO_32(hmmio), MapSL((DWORD)pchBuffer),
+    MMRESULT    ret = mmioSetBuffer(HMMIO_32(hmmio), MapSL(pchBuffer),
                                     cchBuffer, uFlags);
 
     if (ret == MMSYSERR_NOERROR)
@@ -439,10 +439,10 @@ MMRESULT16 WINAPI mmioSetBuffer16(HMMIO16 hmmio, LPSTR pchBuffer,
             FIXME("really ?\n");
             return MMSYSERR_INVALHANDLE;
         }
-        MMIO_SetSegmentedBuffer(thunk, (DWORD)pchBuffer, TRUE);
+        MMIO_SetSegmentedBuffer(thunk, pchBuffer, TRUE);
     }
     else
-        UnMapLS((DWORD)pchBuffer);
+        UnMapLS(pchBuffer);
     return ret;
 }
 
diff --git a/dlls/mmsystem.dll16/mmsystem.c b/dlls/mmsystem.dll16/mmsystem.c
index 1109ef46792..58b46d90b27 100644
--- a/dlls/mmsystem.dll16/mmsystem.c
+++ b/dlls/mmsystem.dll16/mmsystem.c
@@ -814,12 +814,12 @@ UINT16 WINAPI midiOutShortMsg16(HMIDIOUT16 hMidiOut, DWORD dwMsg)
  * 				midiOutLongMsg		[MMSYSTEM.209]
  */
 UINT16 WINAPI midiOutLongMsg16(HMIDIOUT16 hMidiOut,          /* [in] */
-                               LPMIDIHDR16 lpsegMidiOutHdr,  /* [???] NOTE: SEGPTR */
+			       SEGPTR lpsegMidiOutHdr,       /* [???] */
 			       UINT16 uSize)                 /* [in] */
 {
-    TRACE("(%04X, %p, %d)\n", hMidiOut, lpsegMidiOutHdr, uSize);
+    TRACE("(%04X, %08x, %d)\n", hMidiOut, lpsegMidiOutHdr, uSize);
 
-    return MMSYSTDRV_Message(HMIDIOUT_32(hMidiOut), MODM_LONGDATA, (DWORD_PTR)lpsegMidiOutHdr, uSize);
+    return MMSYSTDRV_Message(HMIDIOUT_32(hMidiOut), MODM_LONGDATA, lpsegMidiOutHdr, uSize);
 }
 
 /**************************************************************************
@@ -895,10 +895,10 @@ DWORD WINAPI midiOutMessage16(HMIDIOUT16 hMidiOut, UINT16 uMessage,
 
     case MODM_GETVOLUME:
         return midiOutGetVolume16(hMidiOut, MapSL(dwParam1));
+    /* lpMidiOutHdr is still a segmented pointer for these functions */
     case MODM_LONGDATA:
-        return midiOutLongMsg16(hMidiOut, MapSL(dwParam1), dwParam2);
+        return midiOutLongMsg16(hMidiOut, dwParam1, dwParam2);
     case MODM_PREPARE:
-        /* lpMidiOutHdr is still a segmented pointer for this function */
         return midiOutPrepareHeader16(hMidiOut, dwParam1, dwParam2);
     case MODM_UNPREPARE:
         return midiOutUnprepareHeader16(hMidiOut, dwParam1, dwParam2);
@@ -1010,12 +1010,12 @@ UINT16 WINAPI midiInUnprepareHeader16(HMIDIIN16 hMidiIn,         /* [in] */
  * 				midiInAddBuffer		[MMSYSTEM.308]
  */
 UINT16 WINAPI midiInAddBuffer16(HMIDIIN16 hMidiIn,         /* [in] */
-                                MIDIHDR16* lpsegMidiInHdr, /* [???] NOTE: SEGPTR */
+                                SEGPTR lpsegMidiInHdr,     /* [???] */
 				UINT16 uSize)              /* [in] */
 {
-    TRACE("(%04X, %p, %d)\n", hMidiIn, lpsegMidiInHdr, uSize);
+    TRACE("(%04X, %08x, %d)\n", hMidiIn, lpsegMidiInHdr, uSize);
 
-    return MMSYSTDRV_Message(HMIDIIN_32(hMidiIn), MIDM_ADDBUFFER, (DWORD_PTR)lpsegMidiInHdr, uSize);
+    return MMSYSTDRV_Message(HMIDIIN_32(hMidiIn), MIDM_ADDBUFFER, lpsegMidiInHdr, uSize);
 }
 
 /**************************************************************************
@@ -1077,7 +1077,7 @@ DWORD WINAPI midiInMessage16(HMIDIIN16 hMidiIn, UINT16 uMessage,
     case MIDM_UNPREPARE:
         return midiInUnprepareHeader16(hMidiIn, dwParam1, dwParam2);
     case MIDM_ADDBUFFER:
-        return midiInAddBuffer16(hMidiIn, MapSL(dwParam1), dwParam2);
+        return midiInAddBuffer16(hMidiIn, dwParam1, dwParam2);
     }
     return MMSYSTDRV_Message(HMIDIIN_32(hMidiIn), uMessage, dwParam1, dwParam2);
 }
@@ -1330,12 +1330,12 @@ UINT16 WINAPI waveOutUnprepareHeader16(HWAVEOUT16 hWaveOut,       /* [in] */
  * 				waveOutWrite		[MMSYSTEM.408]
  */
 UINT16 WINAPI waveOutWrite16(HWAVEOUT16 hWaveOut,       /* [in] */
-			     LPWAVEHDR lpsegWaveOutHdr, /* [???] NOTE: SEGPTR */
+			     SEGPTR lpsegWaveOutHdr,    /* [???] */
 			     UINT16 uSize)              /* [in] */
 {
-    TRACE("(%04X, %p, %u);\n", hWaveOut, lpsegWaveOutHdr, uSize);
+    TRACE("(%04X, %08x, %u);\n", hWaveOut, lpsegWaveOutHdr, uSize);
 
-    return MMSYSTDRV_Message(HWAVEOUT_32(hWaveOut), WODM_WRITE, (DWORD_PTR)lpsegWaveOutHdr, uSize);
+    return MMSYSTDRV_Message(HWAVEOUT_32(hWaveOut), WODM_WRITE, lpsegWaveOutHdr, uSize);
 }
 
 /**************************************************************************
@@ -1616,14 +1616,14 @@ UINT16 WINAPI waveInUnprepareHeader16(HWAVEIN16 hWaveIn,       /* [in] */
  * 				waveInAddBuffer		[MMSYSTEM.508]
  */
 UINT16 WINAPI waveInAddBuffer16(HWAVEIN16 hWaveIn,       /* [in] */
-				WAVEHDR* lpsegWaveInHdr, /* [???] NOTE: SEGPTR */
+				SEGPTR lpsegWaveInHdr,   /* [???] */
 				UINT16 uSize)            /* [in] */
 {
-    TRACE("(%04X, %p, %u);\n", hWaveIn, lpsegWaveInHdr, uSize);
+    TRACE("(%04X, %08x, %u);\n", hWaveIn, lpsegWaveInHdr, uSize);
 
-    if (lpsegWaveInHdr == NULL) return MMSYSERR_INVALPARAM;
+    if (lpsegWaveInHdr == 0) return MMSYSERR_INVALPARAM;
 
-    return MMSYSTDRV_Message(HWAVEIN_32(hWaveIn), WIDM_ADDBUFFER, (DWORD_PTR)lpsegWaveInHdr, uSize);
+    return MMSYSTDRV_Message(HWAVEIN_32(hWaveIn), WIDM_ADDBUFFER, lpsegWaveInHdr, uSize);
 }
 
 /**************************************************************************
diff --git a/include/wine/mmsystem16.h b/include/wine/mmsystem16.h
index d30a1ba75d6..05c11be8af1 100644
--- a/include/wine/mmsystem16.h
+++ b/include/wine/mmsystem16.h
@@ -120,7 +120,7 @@ UINT16    WINAPI waveOutOpen16(HWAVEOUT16*,UINT16,const WAVEFORMATEX *,DWORD,DWO
 UINT16    WINAPI waveOutClose16(HWAVEOUT16);
 UINT16    WINAPI waveOutPrepareHeader16(HWAVEOUT16,SEGPTR,UINT16);
 UINT16    WINAPI waveOutUnprepareHeader16(HWAVEOUT16,SEGPTR,UINT16);
-UINT16    WINAPI waveOutWrite16(HWAVEOUT16,WAVEHDR*,UINT16);
+UINT16    WINAPI waveOutWrite16(HWAVEOUT16,SEGPTR,UINT16);
 UINT16    WINAPI waveOutPause16(HWAVEOUT16);
 UINT16    WINAPI waveOutRestart16(HWAVEOUT16);
 UINT16    WINAPI waveOutReset16(HWAVEOUT16);
@@ -139,7 +139,7 @@ UINT16    WINAPI waveInOpen16(HWAVEIN16*,UINT16,const WAVEFORMATEX *,DWORD,DWORD
 UINT16    WINAPI waveInClose16(HWAVEIN16);
 UINT16    WINAPI waveInPrepareHeader16(HWAVEIN16,SEGPTR,UINT16);
 UINT16    WINAPI waveInUnprepareHeader16(HWAVEIN16,SEGPTR,UINT16);
-UINT16    WINAPI waveInAddBuffer16(HWAVEIN16,WAVEHDR*,UINT16);
+UINT16    WINAPI waveInAddBuffer16(HWAVEIN16,SEGPTR,UINT16);
 UINT16    WINAPI waveInStart16(HWAVEIN16);
 UINT16    WINAPI waveInStop16(HWAVEIN16);
 UINT16    WINAPI waveInReset16(HWAVEIN16);
@@ -199,7 +199,7 @@ UINT16     WINAPI midiOutClose16(HMIDIOUT16);
 UINT16     WINAPI midiOutPrepareHeader16(HMIDIOUT16,SEGPTR,UINT16);
 UINT16     WINAPI midiOutUnprepareHeader16(HMIDIOUT16,SEGPTR,UINT16);
 UINT16     WINAPI midiOutShortMsg16(HMIDIOUT16,DWORD);
-UINT16     WINAPI midiOutLongMsg16(HMIDIOUT16,MIDIHDR16*,UINT16);
+UINT16     WINAPI midiOutLongMsg16(HMIDIOUT16,SEGPTR,UINT16);
 UINT16     WINAPI midiOutReset16(HMIDIOUT16);
 UINT16     WINAPI midiOutCachePatches16(HMIDIOUT16,UINT16,WORD*,UINT16);
 UINT16     WINAPI midiOutCacheDrumPatches16(HMIDIOUT16,UINT16,WORD*,UINT16);
@@ -212,7 +212,7 @@ UINT16     WINAPI midiInOpen16(HMIDIIN16*,UINT16,DWORD,DWORD,DWORD);
 UINT16     WINAPI midiInClose16(HMIDIIN16);
 UINT16     WINAPI midiInPrepareHeader16(HMIDIIN16,SEGPTR,UINT16);
 UINT16     WINAPI midiInUnprepareHeader16(HMIDIIN16,SEGPTR,UINT16);
-UINT16     WINAPI midiInAddBuffer16(HMIDIIN16,MIDIHDR16*,UINT16);
+UINT16     WINAPI midiInAddBuffer16(HMIDIIN16,SEGPTR,UINT16);
 UINT16     WINAPI midiInStart16(HMIDIIN16);
 UINT16     WINAPI midiInStop16(HMIDIIN16);
 UINT16     WINAPI midiInReset16(HMIDIIN16);
@@ -440,7 +440,7 @@ LONG 		WINAPI mmioWrite16(HMMIO16,HPCSTR,LONG);
 LONG 		WINAPI mmioSeek16(HMMIO16,LONG,INT16);
 MMRESULT16	WINAPI mmioGetInfo16(HMMIO16,MMIOINFO16*,UINT16);
 MMRESULT16 	WINAPI mmioSetInfo16(HMMIO16,const MMIOINFO16*,UINT16);
-UINT16 		WINAPI mmioSetBuffer16(HMMIO16,LPSTR,LONG,UINT16);
+UINT16 		WINAPI mmioSetBuffer16(HMMIO16,SEGPTR,LONG,UINT16);
 UINT16 		WINAPI mmioFlush16(HMMIO16,UINT16);
 UINT16 		WINAPI mmioAdvance16(HMMIO16,MMIOINFO16*,UINT16);
 LRESULT		WINAPI mmioSendMessage16(HMMIO16,UINT16,LPARAM,LPARAM);