How can I do that? Like the music in Sp you could hear left-right-stereo without any information of the sound-source?s position.
Thanks!

ambient_generic is your solution!

For more information:

http://developer.valvesoftware.com/wiki/Ambient_generic

Hope that helped

More specifically, go to Flags within the ambient_generic properties and check "Play Everywhere". You can trigger it using a trigger brush so that when the player walks through it, the song starts to play.