I've had the silent bug twice, and both times it was after I turned my ringer volume back up from being on vibrate (after a movie or something). Both times, there would be absolutely NO SOUND from my phone at all. No "click" sound when I select something (I have that option turned on), no sound from my Rimshot Button widget, no sound in calls, absolutely nothing.
So the first time this happened, I had just turned my ringer volume back up after leaving work. Then I opened Navigator because I was heading to dinner, to somewhere I'd never been. I got the route set, but there was no sound on the turn-by-turn screen. I checked the volume, and there is actually a "Navigation volume" when you use the volume rockers on that screen, but it was already all the way turned up. Irritated, I restarted, but I still didn't have sound for some reason. I ended up finding out that ALL of the volumes magically turned all the way down. Ringer, media, system... everything. I turned them back up, and everything was fine after that.
The second time was today, after a movie... I had my phone set to vibrate during the movie, then when I got home I turned the volume back up. Later on, I looked at my phone and saw that I missed a phone call and I had a voicemail. Weird, I never heard anything... So I hit the volume rocker, and it was already all the way up. I clicked on stuff, tested my widgets, and sure enough... no sound. Suddenly my phone got tremendous lag, and then completely froze and wouldn't do anything... Then, for some unknown reason, my phone just decided to reset itself... the HTC screen showed, then the Verizon screen, and Sense reloaded... I have no idea why. Everything was fine after that though.