Press the Sleep/Wake button on top of your iPad to put it into light sleep. You can continue to listen to music when the iPad is locked.
The iPad locks automatically if you do not touch the screen for a minute or two.
To awaken your iPad, press the Sleep button or the iPad Home button and then slide your finger to unlock it. If your iPad goes to sleep with the Sonos controller open, it redisplays the same screen it was on when it went to sleep.