I'm having the same problem, looking through other posts with similar issues, it seems to be either a loose connector, should be #5, or the headphone jack assembly, it includes the volume switch, the silent/ring switch, the headphone jack and the lock switch. I took mine apart and didn't seem to be a loose connection, everything else on that cable assembly works fine, thinking it's the switch, but I have to buy the whole assembly to get it....

EDIT: I took mine apart again and messed around with it some, it's not a loose connection, it seems the switch is shorting out or is warped just enough to prevent the required conections from being made inside the switch to go into silent mode. if I take a screwdriver and put any kind of pressure on the switch with the phone apart, it goes into silent mode. I didn't have time to mess around much with it. I got it from not working at all to back to needing alittle pressure under the switch on the outside to go into silent mode, sometimes it stays, but most of the time it doesn't. You would think a case that protects the phone when it's dropped would lessen the effects of the fall enough to keep the switch from warping/breaking but I guess it doesn't. Going to mess around some more later on and see what I can figure out, not ready to spend $50 on something if I cna possibly fix it....