I might add more comments later, as I have a lot I want to say.
FE10 vs Muramasa - Literal coin-flip. Both are very good fits for the game. FE10 is by far the strongest of the series, and I know it better, so tie break with that, I suppose.
SMT:N vs Ace Combat: Zero - Huh. Ok, another one I am very surprised with. Ace Combat Zero has some really solid music that really fits the "flight" aspect of the game. Now, a lot of those songs are awesome, but they are very similar. They all seem to build off the same general theme - not bad, but makes them feel a bit repetitive and more "background noisy". Still, quite close.
Castlevania: Curse of Darkness vs Chrono Cross - I'll echo Niu here. CoD felt a bit weird with the music compared to other titles, though still good. I actually think Gale's decent, though not spectacular. CC is good and I do love a lot of the random tracks. Great variety, overall great quality of every song.
Bastion vs Baten Kaitos - Huh, shit, Bastion's got a pretty awesome soundtrack. Pretty fitting for the game too. Well, a lot closer than I thought. Nom solidarity for one, but...I think I'll go with variety in the fit here. I like the sheer spectrum of the BK soundtrack, whereas Bastion seems to fit in a general line of genre. I'll be sad if BK loses, but Bastion's at least a decent contender.