You can make a sound play when a player enters a certain area, by using a trigger_once entity (this is a brush entity... you basically make a block textured with aaatrigger, and hit the "To Entity" button, then select "trigger_once" from the dropdown menu). Give the ambient_generic that has your sound a name in the "Name" field. Type the same name into the trigger_once's "Target" field. Make sure the ambient_generic has it's "start silent" flag checked... If you want music to play, check the "play everywhere" flag as well. Now, when the player walks through the trigger_once, the sound will play.