Files with extension "5898749c" has the info about which tune is played in the stage.

The value just after the string about the "st_bgm" controls which tune to play. Unfortunately it seems mercs mode is scripted to constantly play one specific song (additional songs will just pile up).

Music list:
1C (28) = b_bgm009 (start of 6-3)
2C (44) = s_bgm020 (mercs music)
2D (45) = s_bgm021 (versus slayer music)
2E (46) = s_bgm022 (versus survivor music)
(54) = bgm007 (mercs stage/character selection music)

Based on st_bgm.stq, the music is in this order:
b_bgm
a_bgm
s_bgm
bgm

-Some info about the st_bgm.stq file:
57 entries of music

-The ogg files has LoopStart and LoopEnd info in each file. It's measured in samples (in the case of s_bgm020, LoopEnd is at 1:35.552 and LoopStart is at 0:06.602667)