Apart from not being able to output multichannel AAC audio (as obviously my AVR doesn't decode multichannel AAC, only AC3 and DTS) I haven't had any problems with audio tracks playing from XBMC on my ATV1 - I've even got an old DVD rip outputting Pro Logic! However, one it does seem to struggle with, is AC3 @ 640kbps! Don't know why, but it just doesn't like it - makes the multichannel light on the front of my AVR go mental and the sound is flitting between on and off rapidly (as in signal > no signal > signal > no signal, etc as far as the AVR is concerned) Don't know why? Restriction of AVR or ATV? Can't be the AVR surely, as the original DVD with the 640kbps DD track plays fine? Likewise, burn the video file to DVD disc, it plays fine from the DVD player. Guess it must be the ATV?