Without looking I am not sure if that is the PSE audio format or the sad/sir0 one.
http://gbatemp.net/threads/the-various-audio-formats-of-the-ds.305167/
Either way tools to directly convert are a bit thin on the ground, though the pokemon people may have made some headway with PSE since I last looked.
If you only want to rip them though you have other options. If the game has a sound test mode then use that, otherwise you get to have some fun fiddling with things.
If you have a point in the game with no sound effects (or background music if you want the sound effects) then great.
If not then even if you don't know how the audio works you can still try replacing files -- overwrite the contents of level_1_theme with final_secret_boss theme and level 1 should be playing the theme you want. Savestates, modded saves, cheats and more can be used to get to points in the game where this is easier. It might get a bit more complex and you might have to overwrite multiple files if the audio is not wave type audio and instead is sequenced, nothing major though.
You could possibly also dip a toe into ROM hacking proper and figure out some things about the format. You need not know everything but if there is a nice volume command or value somewhere you can figure out then that could be very helpful for this (set volumes to 0 and I am sure you can find a use for that). Going a bit further then rather than replacing files you might also be able to figure out the call to play a given track -- super_secret_400_hours_gameplay_boss track is probably just track some_number as far as the game is concerned, such a thing may also dodge problems with different instrument sets or something being used if it is sequenced audio rather than wave type. Not so many examples of audio ripping this way but sprite ripping via emulator does this all the time (why play for dozens of hours when you can cheat or edit a save to give you the end game weapons/character classes/face the final boss in seconds?).
Bonus 2 with emulators is they can usually disable sound channels. Many games will dedicate channels to certain things, that way if the sounds you want are limited to certain channels you disable the others that might be playing ones you don't and there you go. If they are jumping around randomly and you can't keep up (happens enough that I mention it here) you can try doing multiple recordings and overlaying/mixing/dubbing things back in audacity or whatever.
You can also do a lot of this with real hardware, a male-male audio cable and a flash cart if the emulators, or the computers you have available, are not up to the task. It is not impossible to disable channels in hardware but it is more annoying than the simple checkbox in an emulator.
