To extract sounds (or graphics) from a ROM, you’ll need the ndstool and ndssndext.

First extract the game data from ROM:

ndstool -x -d data <filename>.nds

This will create a new directory data containing all the game data. In there you’ll most probably find a file *.sdat somewhere. This is a sound archive format. Now run this through the ndssndext (I had to use WinE):

wine ndssndext.exe sound_data.sdat

This creates a new folder which contains more folders with the actual contents from the .sdat-file. These can be MIDI files and/or (converted) WAV files.