diff --git a/dlls/quartz/acmwrapper.c b/dlls/quartz/acmwrapper.c
index f395f823626..3c6c28ee6ba 100644
--- a/dlls/quartz/acmwrapper.c
+++ b/dlls/quartz/acmwrapper.c
@@ -262,7 +262,7 @@ static HRESULT ACMWrapper_ConnectInput(InputPin *pin, const AM_MEDIA_TYPE * pmt)
 	This->pWfOut = (WAVEFORMATEX*)outpmt->pbFormat;
 	This->pWfOut->wFormatTag = WAVE_FORMAT_PCM;
 	This->pWfOut->wBitsPerSample = 16;
-	This->pWfOut->nBlockAlign = 4;
+	This->pWfOut->nBlockAlign = This->pWfOut->wBitsPerSample * This->pWfOut->nChannels / 8;
 	This->pWfOut->cbSize = 0;
 	This->pWfOut->nAvgBytesPerSec = This->pWfOut->nChannels * This->pWfOut->nSamplesPerSec
 						* (This->pWfOut->wBitsPerSample/8);