The music will scale to the situation at hand. Mods that effect experience, skills, damage values, etc. may alter values used by the audio system and prevent certain tracks from triggering.
I know a few mod attempts were made, but I'm not sure how well they fared. TW3's music system is embedded in the engine, from what I understand, so altering it is nearly impossible.
Could be other things as well. FPS in excess of 60 can cause the frames the system will use to check for audio cues to be dropped. It's also possible that certain tracks can't play if your audio sample rate is too high. Try setting it to 16-bit, 48000 Hz (DVD Quality) if it's not already.
On the whole, I rarely ever got the epic music on my playthroughs. Normally, playing at or around recommended level won't trigger it...and if I did hear it, it usually meant that some wyvern 20+ levels above mine was about to atomize my face. So, I never got to hear it much, nor very often.