Sometimes when I plugged in my dad's Galaxy SII to my PC via USB, the speaker will suddenly doesn't work anymore.
I had tried to play a song/video but no luck. Same goes to incoming call's ringtone.
Tried to perform phone restart, still can't solve it.
I had checked the phone settings, the volume was set to highest level, and silent/vibrate weren't activated, just same as before.

Anyone knows what's wrong with the phone? And how to solve it?

Lock Key + Home Key works too