Here's one (unlikely) possibility Mygoshi could be correct:
* When FFmpeg decodes XMA, it introduces artifacts * FFmpeg translates XMA to WMA without reencoding the content * Playing the resulting WMA file (in a non-FFMPEG player) does not introduce artifacts
The third bullet is probably true. The first two, less likely.
Then you'll have to ask the original game developers to waste more bits on higher quality audio, because FFMPEG decodes it with the "full" quality that is available.
XMA is essentially WMAv2. It is also usually encoded at really terrible bitrates. Encoding to higher bitrates to the same format again *may* mask the original artifacts with all new artifacts, while any other encoder/format would actually try to reproduce those artifacts in an effort to reproduce the "original" (decoded from XMA) signal.
But the problem is that I can't hear any bad quality/artifacts when playing Xbox 360 games you know. There's no solution to get a converted file that sounds in game? You will say that games uses filter or some tools to increase the quality but no, not of all games use this, and they even don't have any artifacts.