Okay so I ended up figuring this out myself. The music you set via Music Volume Trigger is not just an ambient music track. Each entry has a specific ambient track as well as a combat track assigned to it and the two do not seem to be seperately editable.
So for example: if you want your combat to play the shadow prince combat theme, set your Music Volume Trigger to play Boss_ABMS_Orch_music_on (which does not support the four instruments) or another example: explo_theme_16_dark which plays 'dancing with the source' (which supports the four instruments)
Ideally there would be a wiki page with a table of what every entry plays but for now looks like it's either trial/error or going into the game levels to find the one you want.
