In Windows Mobile, If a Bluetooth headset is connected to your Smartphone, you cannot put the phone in silent mode. You will need to manually disconnect the pairing by disabling Bluetooth or powering off the headset. Again, if you’re not using a Bluetooth headset, you must still change the profile to Silent manually.

LightsOut is a freeware application for Windows Mobile Smartphones that detects the insertion of AC adapter in your mobile device and disables Bluetooth as well as puts the smartphone in Silent mode automatically. Upon removal of the AC adapter, Bluetooth is restored and the profile is changed to the previous state.

Via MSMobiles

Download here