Features in Multimedia
Add MP4 replay gain tag support, providing you have libMP4v2.
If you don't have libMP4v2 and want MP4 tag reading support, you either need to install it or extend metadata/m4a to read freeform data (ie: starting with ----).
I personally have no interest in working on metadata/m4a when metadata/mp4 works perfectly well. However, if someone implements freeform data reading in metadata/m4a, I'm happy to do the rest of the work for reading replay gain tags.