Waiting for all sounds to finish is straightforward: sleep["play"]
So, playing a sound during a transition, letting the sound finish, and then playing another might have a script like:
play["firstSound"] go[anotherCard "BoxIn"] sleep["play"] play["secondSound"]
Decker isn't really designed with looped audio in mind. Keep in mind that individual sounds in Decker are capped at 10 seconds, so they wouldn't work especially well for background music. You might be able to get continuous background noise by monitoring sys.ms and periodically issuing another play[], but it isn't straightforward. Perhaps a feature for the future...