For any MusicBee users who have MP3 files specifically that sound too quiet, the issue is the VOLUME_ADJUSTMENT tag. I’m not sure what creates this tag, as MusicBee itself does not (tested just yesterday with some MP3s). I suspect that it’s probably Quod Libet adding this – I had used this library on Linux through that in the past before going back to MusicBee.

MusicBee doesn’t create the tag, but seems to use it if it’s present, and then things play too quietly. For any affected MP3 files, just remove the VOLUME_ADJUSTMENT tag. Then your MP3s will sound louder again. If you have any ReplayGain tags in the files it shouldn’t be affected, MusicBee will read these tags as normal.