IDJC doesn't respect replay-gained MP3 files that have their RG data only in the ID3 tags (i.e., not using APE and/or LAME).
The code in python/playergui.py is basically correct but ignores the fact that Mutagen's EasyID3 class will NOT, per default, read all tags. To read the "TXXX:replaygain_track_gain" tag correctly, the key must be registered with EasyID3 first.
See attached diff.
(We might be calling RegisterTXXXKey unneccessarily often here, maybe the registering could be moved elsewhere?)