The big gaps in the list are due to the fact that you can add or replace music in SRB2 by importing it into a wad and naming it O_MAPxxM or D_MAPxxM, where xx is the map number. Use D_ for MIDI format music and O_ for everything else. Currently, only Ogg, MP3, Wav, MIDI, MOD/S3M/XM and IT are supported, though it is suggested that you only use Ogg and MIDI.