Use your Mac's volume keys to adjust the volume when iTunes is streaming through AirPlay.
FromCordless Dog:Streaming iTunes via AirPlay is great. Having to switch to iTunes to adjust the volume is not. AirVolume makes this process seamless. Ordinarily, the volume keys on your Mac's keyboard adjust the system volume. AirVolume uses your volume keys to adjust the volume in iTunes, but only when iTunes is playing via AirPlay