1. Start World Editor (WE) 2. Load your map 3. Press F5 to bring upp the sound menu 4. Press CTRL + i 5. Import the sound you want. 6. Scroll down til you can see the "imported" category 7 Right click and "Use as Sound"
Don't forget that if you want to use the map at B.Net, the map cannot be more then 4mgb.