diff --git a/dlls/winmm/tests/mmio.c b/dlls/winmm/tests/mmio.c
index 5e440b8890b..57dc1668a28 100644
--- a/dlls/winmm/tests/mmio.c
+++ b/dlls/winmm/tests/mmio.c
@@ -608,6 +608,10 @@ static LRESULT CALLBACK mmio_test_IOProc(LPSTR lpMMIOInfo, UINT uMessage, LPARAM
         return MMSYSERR_NOERROR;
     case MMIOM_CLOSE:
         return MMSYSERR_NOERROR;
+    case MMIOM_SEEK:
+        lpInfo->adwInfo[1]++;
+        lpInfo->lDiskOffset = 0xdeadbeef;
+        return 0;
     default:
         return 0;
     }
@@ -632,6 +636,10 @@ static void test_mmioOpen_fourcc(void)
     mmioGetInfo(hmmio, &mmio, 0);
     ok(hmmio && mmio.fccIOProc == FOURCC_XYZ, "mmioOpenA error %u, got %4.4s\n",
             mmio.wErrorRet, (LPCSTR)&mmio.fccIOProc);
+    todo_wine ok(mmio.adwInfo[1] == 0, "mmioOpenA sent MMIOM_SEEK, got %d\n",
+       mmio.adwInfo[1]);
+    todo_wine ok(mmio.lDiskOffset == 0, "mmioOpenA updated lDiskOffset, got %d\n",
+       mmio.lDiskOffset);
     mmioClose(hmmio, 0);
 
     mmioInstallIOProcA(FOURCC_XYZ, NULL, MMIO_REMOVEPROC);