2.3 introduced the ability, by pressing "pause" 3 sec, to access a menu "chapters and audio tracks" ; there is an issue with audio track selection : I made a .m4p with 2 tracks (english + french) ; when played, video was OK but no sound. Audio track menu showed both tracks "deactivated" ; I select "french" for example, and I get then the "french" track (although it keeps showing "deactivated") ; I switch to "english" : it works ; but from that point, impossible to switch back to "french", I have to stop the movie, select it again and then the selected audio track is "french".
Generally speaking : 2 audio tracks (maybe more) means no sound when movie plays first time; you have to select manually one ; if the selected language is "english", you wont be able to switch to another language without stopping first the movie and replay it.
Something else : the sleep mode, which was accessed by pressing "menu" more than 6 sec (before 2.2) disappeared in 2.2, and was supposedly reintroduced in 2.3 ; it was, but only on the main menu ; it used to work everywhere ... which was very useful when you had your TV shut down and wanted to send your ATV to sleep mode, too.