You can put your Windows Phone 7 to silence mode by following the steps below

How to Turn off / on the Ringer in Windows Phone 7 ?

1. Tap / Click right arrow in the start screen to navigate to the list of Apps .

2. Tap / Click the Settings in the Application list

3. Tap / click the Ringtones + Sounds option to launch the Settings Form .

4. Switch On/off the Ringer option accordingly .