Basics for a sound wad:
1: You need a .wav file.
2: You need a wad editor. I recommend
XWE.
Now, open XWE (after installing).
Go to: File > new.
Save your new wad wherever you want it.
Now:
You need to edit the properties of your .wav, so that it plays at 8 bit mono. If it already plays at this, you may skip this step.
Open Sound Recorder.
Go to: File > Open, and open the .wav file.
After you open the .wav in Sound Recorder, go to: File > Properties.
A new window will pop up. Click on the button: "Convert now".
Now, a second window should pop up:
There will be 3 sections.
Ignore the 2 that are named:
Name, and Format.
Click the little popdown menu next to Attributes, and scroll down (or up) until you see this setting:
48.000 kHz, 8 Bit, Mono
Click that.
Click Ok on the two popup windiows (This one, and the one before it).
Now go to File > Save as
Save the new .wav in any folder you like.
Now:
Go back to XWE, and open the wad file you made earlier.
(File > Open)
After opening the wad, go to Entry > Load, and load your .wav sound.
Now, Open XWE again. You want 2 of these open. Open SRB2.SRB
(Go to the folder where this is installed, and type it in)
And click the sounds button, at the bottom)
You should notice a new option at the top:
Wave.
Select a lump (file in a wad, all the bunches of random text to the left of XWE)
And click Wave > Play.
Keep doing this for each lump, until you hear the one you want to change.
Now, go back into the XWE with your wad in it. Rename the lump to the one you found in SRB2.SRB.
Now, there's one more step:
Click Wave > Save as Doom Wave. (The one with your wad.)
That's it.
Close XWE, and load the wad into SRB2.
There's your new sound.
Edit: You wanted music?
Well, you can use MP3s, or OGGs.
Do the same thing: Use XWE.
You can skip the Sound Recorder section.
Open the OGG/MP3, in XWE and rename the lump.
Like this: O_MAPxxM
(Replace xx with the level number. Bosses all use level 3. If the level is only a one digit level, use 0 before the digit)
So, if you want to replace the GFZ music, the lumps would be named:
O_MAP01M
O_MAP02M