Before you complain about not finding the sound file in the source engine files you need to do it by yourself, that means that first you need to make a folder name "sounds" after that enter the sound file and make another file called "lambdaplayers" and inside the lambda folder you have to make another folder named "musicbox" and inside that folder you can finally put your music in the indicated sound formats, if you only want to hear the music you put in the musicbox folder in gmod, you need to check if the opcion "custom music only" is activated in the lambda players seccion.