Felt the exact same. Only played the trial due to lack of funds but that was the one thing that stood out to me!

I played this game with the sound turned all the way down and my own music on so I don't notice.

The music isn't bad, but it's mostly generic. I agree they should have put more into the music... but I don't think the game is unplayable by any means due to the music choice.
 
What would you choose as a musical style for the game? Metroid had funky space ambiance and SOTN had mostly baroque-ish tunes.

agreed... but it probably sounds so nice because it's a change from what you're hearing throughout the rest of the game.

I quite liked the piano music you had when you went back through the section of the map that you flooded.

I didn't find this to be a problem. You see, the music in the Castlevania and Metroid games is accompanied by outlandish environments, something Shadow Complex lacks. In the grey, industrial-looking environments found in SC, the ambient sounds(and the occasional short pieces) work perfectly.

Yeah, the game lacked in music, even a title-melody, I can only remember the piano-scene. Still a great game though, even though it was so grey, brown and black all the time. Would have been cool, to see it with a more unique style

Yeah the music kind of sucked, though the underwater part with the piano slowly fading in was nice. It really set the mood for that sequence and I wish they would have done that more to really add an emotional impact to the fact Jason is killing hundreds of people.