If they are already using iTunes with an AppleTV, Airport Expresses could be used around the house, attached to speakers. I've had iTunes outputting to the Mac, AppleTV, and 2 Airport Expresses without any issues.

Also, some receivers have some nice wireless speaker output options. I bought my mom a Sony receiver along with an iPod dock, and it came with an extra set of speakers that work anywhere in wireless range of the receiver. The extra wireless speakers are between the size of a large alarm clock and a small boom box.