You'll need some sort of trigger (a button, invisible place on floor, etc) to start the music.
Make a folder in the sound folder of your mod.
Name the folder whatever you want.
Find the wav file you want (has to be 22.025hz 8bit mono)
Name the wav something simple.
Place the wav in the folder you made.
Now place an ambient_generic in your map where you want the music.
Name the ambient.
Browse to the wav file in the folder you made.
Set it's properties.
Flags: start silent / not looped
Now make the trigger to start the music.
Have the trigger target the name of the ambient.
Set time limits, making sure it won't overlap itself.
Hope this helps.