Vgmtrans vgmtoolbox to to unpack the music after you unpack the rom with something else(dslazy, dsbuff)
play the files vgmtrans gives you til you find what you are wanting to replace - use trimming and converting software to edit your music to fit in the same space size as of that which you are replacing hope vgmtrans can help you compress said music and then use dslazy dsbuff to rebuild rom
complications
1 there may not be software to convert your music to the type the game uses by vgmtrans or thats in the game itself
2 you will have a lot of trial and error getting this to work and even then very doubtful
3 (note most undubs are done with games that are similar the same and they're music files are pretty much identical)
4 MOST IMPROTANT i don't know if VGMtrans VGMtoolbox will compress back so...
Current vgmtrans can be found at sourceforge.net and dslazy dsbuff can be googled and gotten that way
use the faqs here and at romhacking.net to see how much more complicated this actually is.
Different games have different music files
GOOD LUCK (giggle)