Those USB sound cards are just a very cheap and easy way of adding audio zones, and you control it like you would an MD. I have one for my bathroom speakers, one for my garden speakers, and one for my guitar amp. They are brilliant considering it means you can do away with an entire MD and its associated gubbins if audio is all you need.
I don't use Pandora (not sure I really know what it is), but why don't you have a look at adding some internet radio stations and playing those through your new USB sound card.
Cheers,
Matt.