I had the same problem - no audio playback on encoded AVI's with AMR encoding, but fine with WB-AMR. The problem seems to occur when the original file audio format is PCM 8kHz 16bit mono but not for other formats (e.g. PCM 44kHz 8bit stereo).
Other audio-only file formats such as WAV seem to encode OK.
I also noticed that the playback volume on the Media Converter can be affected by the Nokia Multimedia Player volume setting if both are active at the same time.
I had the same issue as vidiman.
Supposedly the specs say that to convert a wav file to an AMR file, the wav file needs to be an 8khz, 16bit, mono file. But when you convert this format to an AMR, there is no sound. If I change the wav to an 11.025khz, 16bit, mono format and then convert it to AMR, it works perfect.
Not sure if it's a mistake in the program or what but I did the same conversion on 3 different pc's and got the same results on each.