Forgot to mention that I've tried that also and it doesn't work.What about disabling MP3 in ffdshow audio codec settings?
Did filmerit find any errors, strange that DMO filters are not being used.
What I would do is remove both ffdshow + AC3, remove any saved setting by reverting to default before doing this, then check with filmerit, then run ccleaner on the registry.
Without installing any codecs do mp3's play in MP, if so install ffdshow and check again, if ok, then install AC3 and check yet again.
Did filmerit find any errors, strange that DMO filters are not being used.
What I would do is remove both ffdshow + AC3, remove any saved setting by reverting to default before doing this, then check with filmerit, then run ccleaner on the registry.
Without installing any codecs do mp3's play in MP, if so install ffdshow and check again, if ok, then install AC3 and check yet again.
If you had not stated that mkv's work I would say, you have accidently muted MP in volume mixer, can you just check to be sure.
Start MP, then use alt-enter to get to windowed mode and play music, then mouse over to volume icon in systray, click on this and then go up to mixer, you will see Mediaportal listed, does it show anything different to master volume, ie muted.
Logs show no problems.