I just went though the CEA campaign again the other day, because my mate lent me his 3D TV, and couldn’t help but notice how quiet the dialogue was in comparison to the music.
A points I couldn’t hear what Cortana was saying because the music was already to loud, and turning up the volume would only make the music louder still.
I looked in the menus, but couldn’t find anyway to adjust the levels individually.
Has anybody else noticed this?
YES! I have noticed this and it’s quite annoying. Even with studio headphones, it’s hard to hear.
I think since because all of the music was re-done (not remastered, but re-recorded) and the dialog audio was just remastered and never sounded that great in the first place (in terms of quality) they tried to hide that by blending it with the music, but ended up drowning it.
You can bump up the mid-level EQ to try and bring it out a bit, but it only helps a little.
Yes and it’s incredibly annoying. I love the music in Halo but not when it’s louder than the dialogue I need to hear to understand the story.
I hope the same mistake isn’t made with H2A.
Also, that one music glitch at the end of the game where it continues to play the Pillar of Autumn music while the final cutscene plays is horrible. 
I have always had the opposite problem, where the in-game audio completely overpowers the music.