For software I generally recommend SLADE. All music files can be found in music.dta, and all sounds can be found in srb2.pk3. For sounds: to my knowledge, modifying srb2.pk3 will prevent the user from being allowed to join netgames, so you will want to create a new wadfile and place all of your sounds in that, rather than editing srb2.pk3 directly. However, you do have the option to modify music.dta directly -- netplay does not check music.dta or other wadfiles for consistency if there are only sounds inside and/or a music playlist file inside (MUSICDEF).
(Note: SLADE and other lump editors may not show music.dta in the file browser as a "supported" filetype, but you can still type in music.dta in the filename field and the program will read and open it.)
At this point, I'm not even sure what filetypes SRB2 doesn't support. You can add wav, mp3, ogg, mid, it, nsf, vgm, etc. and chances are the game will play the audio.
