The latest version of MediaMonkey creates ReplayGain (volume analysis) tags which Rockbox 3.7 cannot understand.
Ref. These example files. http://www.goodbit.net/martyg/MM-Replaygain.zip
This is from the same source file, processed through v1297 and v1304 of MediaMonkey.
Rockbox recognizes the tag embedded in 1297.mp3 and adjusts level accordingly.
Rockbox cannot recognize the tag embedded in 1304.mp3.