I have an iPad 1, and the mirroring came with iPad 2 and iphone 4S.
The iPad 1 supports Airplay from within an app which will send an often pre-rendered video feed via the app to AppleTV.
Airplay mirroring is different in that the actual screen display is processed and compressed and relayed to AppleTV rather than sending video footage of some kind.
If you only get audio from within an app that is usually because the app supplier limits Airplay to audio only - this applies to many big media companies. They do not offer video Airplay presumably for licensing reasons that are beyond our comprehension! Check the support site for the app.
While iPad 1 might be able to manage Airplay mirroring after a fashion Apple will presumably have decided it was not up to the standard required to enable the feature (it may not have relevant hardware).