There are basically 3 options for simple wireless audio:
1. RF transmitter/receiver combo
2. Bluetooth transmitter/receiver combo
3. Sonos setup
Assuming you're outputting analog stereo (left & right white/red RCA plugs or a minijack port), it's pretty easy. First option is to go with a basic radio streamer. There's a bunch out there, none are 100% awesome though (read the reviews). However, there is one here with pretty decent reviews for $60 shipped:
http://www.amazon.com/Nyrius-Transmitter-Streaming-Wirelessly-NY-GS10/dp/B00FB1NTP6/
They sell a second receiver for $47 shipped:
http://www.amazon.com/Additional-Nyrius-Separately-Streaming-Satellite/dp/B00GGQGKFQ
The next question here would be input...would you be looping the outputs into your computer via the input jack on the sound card or what?
Anyway, moving on - you can also do a Bluetooth setup. I have an Anker Bluetooth combo transmitter/receiver unit & there is some lag if I watch TV with it (my receiver has an option for delay sync). There's a bunch of other similar units, like this one for $14 shipped:
http://www.amazon.com/Bluetooth-Receiver-Patekfly®-Hands-free-Streaming/dp/B019Q1PWX8/
Not sure if you can pair more than one to one tho...I know with those portable Bluetooth speakers, the newer ones let you pair two of them for stereo use, but that's off like a smartphone rather than a transmitter, so that would require some research.
Third option is to get a Sonos setup. No lag, works awesome, downside is that they are very expensive ($350 for a base station with no amp).