[Problem]Background Music Lightness

[Silverthorn]

Probably because their amplitude isn't as high as it should be.
Get a wav-editor (there are loads of free editors out there) and open any original bgm in it. Now, check how big these....

things (Click to View)

...are.
The bigger, the louder. Now, compare those to the bgm you want to integrate into the game. Probably, you'll have to normalize the file (the max. amplitude will be rise to 0dB which is the highest you can get without making your sound card go nuts). Increasing the level by a couple dB should work well, too. Find out what's best for you