Hello now as far as adding music goes you can do that BUT I only know how to add them from other DS games which is what I did with my team CV3 hack, from what I've seen ALL of the music tracks from DS games can be used so while you may not be able to add completely new music ingame you can in fact added different tracks from other games since I tested it out with games like Mega Man ZX and Super Mario Bros (just for fun) and all those games worked with this method.
1.Step one is getting the game extracted which is from what you said is Dawn of Sorrow which after you have chosen to extract it using DSvania tool you will get a folder full of content for it, afterwords download a program called DS Sound Studio located here
drive.google.com/file/d/0B9ZYBSGvTGbiSWJUYWYyNy1idkU/view, after you have extracted the program within its own folder then open DS Sound Studio and then navigate to the folder where you extracted Dawn of Sorror in and choose to open the file called sound_data.sdat and if you have done it right you should get a list of all the music tracks for you to listen to and replace if you want later.
2.Next step is to now choose which tracks you want to replace, listen to them within DS Sound Studio by double clicking on them and pressing play, after you find the track you want to replace close the listen box and then right click on it and choose Import from SDAT, the next step is to navigate to your other extracted game which as I mentioned earlier can be done with DSvania if its one of the DS Castlevania games but if you want the sdat sound file from another game you use a programe called Tinke located here,
github.com/pleonex/tinke/releases, like DS Sound Studio make a new folder and extract that Tinke programe there and then open it and navigate to any Nintendo DS rom you want music from and once its open find the Sound Sdat file (which normally has a music symbol) in the file list and use Tinke to extract that sdat since the sdat files are the main thing your going to get your music from.
3.Final step is to open whichever Sound sdat file you want music from within the import sdat section of DS Sound Studio which I asked you to do earlier, once you have done this it will pull up a list of different music tracks for you to choose from within the "Sequence" section, find which one you want and then choose OK and afterwards save using DS Sound Studio but DON'T close it yet, now go back into DSvania and then choose to save and build the Dawn of Sorow rom who's sdat you just edited and once your done test that built from out ingame to see how it sounds and your done.
Any further questions or help you may need feel free to ask.