First hacking sound files is not as easy as some of the text and other things, I highly suggest you become familiar with basic DS formats and similar before venturing too far into it.
How do you extract the sound files from an sdat file?
Many methods.
crystaltile2: thread in this section somewhere.
ndssndext:
http://www.4shared.com/file/68276816/80922...sndext_v04.html
vgmtrans:
http://www.4shared.com/file/76520093/5a299676/VGMTrans.html
Other methods but they are older and a bit more fiddly.
How do you convert/change mp3 files to the DS friendly types?
Assuming you are dealing with SDAT which most games use (some like Electroplankton use wave and other games use or other formats). Most of the time you do not, still the DS uses PCM format (sometimes also ima-ADPCM) files which form the instruments and several other files, get the PCM in the correct format and insert it into the appropriate header (I usually get and existing file and tweak it)
http://kiwi.ds.googlepages.com/sdat.html for info on sdat.
Midi files (like sseq) can be converted.
http://kiwi.ds.googlepages.com/
Owing to the somewhat imprecise methods used as well as the DS hardware I would not expect much.
Which types do I need to change the mp3s to?
MP3 is a representation of a wave file, midi and the DS counterpart SSEQ which represents the vast majority of DS sounds are instrumental/tracker formats depending on how you look at them.
I know of no good way to convert wave files to midi (the other way is OK) and have them sound nice, however I am far from an audio expert and lots of stuff has happened in the last few years so do have a look.
How do I repack it?
Repack SDAT, there is an early stage repacker but I do not like it. Head over to the tahaxan forums and look at the posts from kiwi.DS (the guy behind a great deal of the sdat work for the DS) if you want it.
I use a hex editor, cat or copy /b, crystaltile2 for info on sdat files, patience and
http://kiwi.ds.googlepages.com/sdat.html when doing such things.
Repack DS: if your SDAT file is the same length then use ndsts or ndshv (nds header viewer) to replace them, if it is smaller I would pad it and also add it in.
Bigger unless you want to cut the SDAT file (which may well defeat the object of hacking it depending on what you did and how you did it) then it is repack with ndstool (frontends DSBuff and DSlazy if batch files are not for you) or a similar method (nitroexplorer or something).
How to replace the files after changing them?
Covered in repack DS above.
Other notes
Most people do not replace files with brand new files but take them from other versions (undub beyond the basic swap a file from one region to the other) or other games (a bit harder but not impossible by any means, remember to take instruments when dealing with SSEQ) or other parts of the file (see the tetris sound hacks, I made a guide on a megaman sound hacking thread a little while back and there have been several other hacks using those sorts of methods). Or they use such techniques to increase stability but effectively disabling the sound and freeing up resources.
