I never went in-depth with sound files, but I'll explain what I know.
Most sounds in the game aren't going to be what you think of when you think of sound files. For instance, in an MP3 you have one long stream of information that is a direct representation of sound(waves). In an effort to save space, and because most music for the DS is synthesized anyways, sounds on the DS are more like a MIDI than an MP3. MIDIs, instead of storing direct sound data, store information that tells a set of virtual "instruments" what to sound like in a sequential fashion. An SSEQ is equivalent to a MIDI, in fact you can convert using some neat tools. SBNK files are the equivalent of "sound fonts" for MIDIs; a set of digitized instruments. AFAIK there's no existing tools to make a sound font from an SBNK.
SWAVs will also be in there, these ones are more similar to MP3s (or if you're familiar with the format, WAVs, heh). I know this is used for stuff like pokemon cries where it wouldn't make sense to use any kind of instrument sequence. Any other files you see are just gonna be some archive format containing the formats I mentioned.