For the SA wads, go on the official website and look in the addons section.
For the custom music, (I only know how to do this with DeePSea) do the following:
1.get an mp3 to ogg converter and convert your mp3 to the ogg format. If you want to add a custom midi and not an mp3, skip this completly.
2.Rename your music. For MP3's and OGGs, you rename it O_MAP*number*m. If you don't have a hex editor, your map will automaticlly be map01, therefore you should rename your map O_MAP01m.ogg. If you have a midi, you rename your music D_MAP01m.mid.
3.Open DeePSea up.
4. Press F7.
5. Click on the button that says "Merge-inport Multiple files into a PWAD Import Graphics, Mustic, and anything else".
6.Click "Open Target File".
7.Open your wad.
8.After you have opened your wad, Click the Button "Add Lump Files".
9. Load your lump file. In this case, your lump file is your custom music. Lumps are really any kind of custom addon to your level. These can be Music, Textures you ripped or made yourself, and many other things.
10.Click the button save all files.
11.It will now say something along the lines of insert all files after Map01. Click yes.
12.Exit and check out your level.