I was honestly hoping you would use the search function and look for it yourself... As there are a few threads on sound extraction for the ds.
Otherwise, it depends on the type sound file in the game. open the game up in tinke and look for sdat, ahx, adx, sad or anything in a "sound" or music folder. You may need to familiarize yourself with the other nds rom file formats so you can distinguish what you are looking at. you can also (after you get the sound file) use something like vgm tool box to convert the sound file after... but try Tinke first. I may post more later...