Well the music and sound files are already on anybodys computer who already has the game. The assets files are zipped up into a pk3 but essentially They are still physical folders in your base directory.
So to play a music file that came with the game you would just use a command like this
And you just use that either in the worldspawn or a "target_play_music" entity.
Thats the path that's located in the pk3. So a good thing to do if you want to listen to the files and get the correct paths is to unzip the assets file into a seperat folder just to help you to find the correct path.