You can check Joshw archives and HCS forums to see if old games rip haven't already been done :
https://www.hcs64.com/mboard/forum.php?showthread=26929
https://hcs64.com/mboard/forum.php?searchmode
http://dsf.joshw.info/k/KISS%20Psycho%20Circus%20-%20The%20Nightmare%20Child%20(2000-10-29)(Third%20Law)(Tremor)(Gathering%20of%20Develope rs).7z
Use Foobar2000/Winamp with the correct component/plugin to read and convert those kind of file. Vgmstream is a good one for old stuff, but there are many other ones for DS sound files etc ...
You could use the Foobar on my sig, it already have some usefull components + encoders to convert them in mp3/flac ...
convert to either flac (lvl 5-8) or mp3 v0 if you don't want Lossless