First of all, I'd like to congratulate you on your efforts! The game is looking awesome and doing everything by yourself is certainly no easy feat.
Regarding the music, I must agree with mrmixer. In general, the melodies are too "happy", which I think doesn't fit the mood for each part of the game.
For example, the "Mines - Grotto" theme (1:28) starts really well, giving an eerie vibe but then you go to C major and it loses that mood and becomes too playful for the setting. Maybe you could develop a bit more that initial theme with the marimba and the voice and add more layers, trying to avoid melodies in major keys so that it doesn't sound so "joyful".
I took the chance to put together a short example based on that track to illustrate what I'm saying. If you have any questions, feel free to ask, I'll be glad to help!