I had the exact same issue almost from the day I got my Surround (launch day). Plugging anything into the 3.5mm jack, headphones, aux speaker cable, anything), especially while music was already playing, would send my down to 0. If I tried to move it back up with the volume key, it would fight me and move back down again. Also randomly, TellMe would popup when plugging in, asking me to confirm a voice command. One of my few WP7 embarrassing moments when trying to play music from my phone through aux speakers at a friends Christmas party.
Originally Posted by Ruined
I did a factory reset on my phone a couple months ago and since then the problem has completely vanished, along with an alarm bug I also had. Using same headphones and repeated connecting/disconnecting has no ill effects. So I would suggest trying a reset. Good chance it will clear up the issue for you.
Sent from my HTC Surround using Board Express