There are two scripts that come with FPSC made for sound zones. They are "soundinzone.fpi" and "soundloopinzone.fpi". The first will play a sound when the player goes into the sound zone and never play it again. The second will loop the sound while the player is within the zone. When the player exits the zone, it will stop playing, but will re-play the sound when the player goes back into the zone.
It sounds like the second script is the one you're looking for so put that into your sound zone's Main AI field in its properties.
The one and only,