MP4 Audio Issues

[jimbo6000]

Hello all,
I upgraded my amplifier the other week and again discovered I had problems with audio and MP4 files. With my other amplifier I had no sound so I set AAC and LPA (?) settings to "Decode to PCM" and this seemed to fix the problem.
This time, I have all audio working apart from the dialogue. So I went back in to the audio settings and set AAC and LPA to "Decode to PCM".
This seemed to work but when I came back to the movie the next day the dialogue dropped again and I cannot for the life of me get full audio when playing MP4 files. I checked the audio settings and AAC and LPA were still set to "Decode to PCM".
Anyone experienced this?
Cheers,
James