Hi. First of all thanks for adding support for libmp4v2, it seems to work fine. However, I like to sort my music by album artist/compilation and the current implementation does not allow this (afaik). Attached is a patch for libmp4v2_handler.cc that allows auxdata for m4a files to be accessed in the same way as files tagged with id3. I did it this way to avoid messing around with how the configuration file is parsed and to make it easy for people with mixed mp3/aac files to use. Currently implemented frames are as follows:
TCOM - corresponds to 'composer'/'writer' tag
TENC - corresponds to 'tool' (ie iTunes 8.0, faac etc)
TRCK - track number, which is formatted as a string "track/total" if such information exists
TPOS - disk number, formatted in the same way
TIT1 - grouping tag
TBPM - bpm/tempo tag
TCMP - compilation flag ("1" if set, "0" if not)
TPE2/TPE4 - both set to album artist tag
However it would be relatively straightforward to add more, eg. to make TPE3 point to album artist just add ' || desiredFrame == "TPE3"' to the if statement before MP4GetMetadataAlbumArtist, or to create some aac-specific naming scheme if you want.
I hope this can be merged with trunk if possible, and if you have any issues you want me to clear up (eg. documentation) please let me know.