The volume of the keyboard's Main Voice is controlled by the "M.Volume" function. There are also "D.Volume" and "S.Volume" functions for the Dual Voice and Split Voice.
The overall volume of the Song is controlled by the "SongVol" function. The volume of the individual MIDI channels for the Song are set inside the Song and cannot be changed on the keyboard, but "SongVol" adjusts them as a percentage (I think) of their individual volumes-- so setting "SongVol" to 100 would set each channel of the Song to 100% of the volume as specified within the Song file itself; 50 would set it to 50%; 127 would boost it to 127%; etc. If you want to adjust the actual channel volumes as specified within the Song file itself, you'll need to use a MIDI editor on your computer to edit the Channel Volume messages in the file for each channel, then save the changes to the file.