At this point I would accuse Heran Bago of overthinking it.
1) Back up rom.
http://www009.upp.so-net.ne.jp/rudolph/nds/NDS_Backup/
2) Disassemble:
http://ezflash.sosuke.com/viewtopic.php?t=457 (this is more pull apart the file system for step 3). There are many other methods but a simple batch file works for me.
3) Sound format: Assuming it is SDAT format (and disregarding region dupes I can still count them on one hand) then it has well understood for quite some time.
4) convert mp3s to a more ds-friendly format. The hardest part and most likely where you will come unstuck.
There are multiple formats used by the DS but in reality they amount to three things
Wave files: whole songs and audio clips
Instrument files: normally notes rather than clips.
Midi type files: play short clips or instrument files.
Wave files are usually a form of PCM or ADPCM audio (well described in the link above)
SSEQ: midi and can be made
http://kiwi.ds.googlepages.com/midi2sseq.exe
The rest you will have to make by hand. I am not sure how well the rom will take to you changing "midi" to wave files either (I imagine it would be dropped from the rom if not needed which means you would have to go through the steps from Heran Bago).
Rebuilding the sound file: the hardest part. There are early tools but for a project like this it will be build your own or sit there with a hex editor for some time.
Early stage rebuilder:
http://tahaxan.arcnor.com/index.php?option...&topic=68.0
Putting the DS file back together was covered in the link at 2)