Wow, I can't believe this. I think I know why the music sucks- it's because nobody from the previous games worked on this. Harry Gregson-Williams was only the music producer, hence he didn't provided any tracks to the cutscenes like previous games. The rest of the music is composed by Ludvig Forssell & Justin Burnett, both without any credits to their belt.
I don't know how the game will turn out, but one thing's for sure is that the music is unremarkable for a Metal Gear game. It makes up for that in (awesome) 80's pop songs, but as far as original compositions go, this game has nothing good. Gone are the days I listen to lossless Metal Gear music while driving to work.