Selecting multiple speakers (devices)
It is possible to play iTunes songs on your home’s AirPlay compatible speakers (devices) other than those of this unit.
Click the AirPlay icon 
and select “Multiple”.
and select “Multiple”.Check the speakers you want to use.
NOTE
In playback using the AirPlay function, the sound is output at the iPhone, iPod touch, iPad or iTunes volume setting level.
You should turn down the iPhone, iPod touch, iPad or iTunes volume prior to playback and then adjust it to a suitable level.