U have a few options depending on your needs.
One would be to download apples free "remote" app that will control iTunes on your computer through wifi, which you can leave attached to your speakers. (def available for iPhone, imagine should be available for iPad)
if you want your speakers in a different room/place to your computer get an airport express and plug your speakers into it. You can then play music in iTunes wirelessly from your computer, which can in turn also be controlled by your iPad via the previously mentioned remote app. The wireless music is however limited to just music through iTunes.
http://www.apple.com/airportexpress/features/airtunes.html
The next option extends on the standard capabilities of the apples airport express allowing you to stream audio from a sources other than iTunes. Haven't checked the feature set in ages but I believe they've entered the app store and might offer something you need.
Check out
http://www.rogueamoeba.com/airfoil/