Are sure your not accidentally hitting the volume buttons. You can change the volume with the screen locked so turning off the screen or locking the phone won't keep this from happening.
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.I am having a bug where the volume will inadvertently go to 00 or 30 for no apparent reason when listening to music and surfing the web at the same time. Anyone else have this? It then sometimes not allow me to lower or raise the volume when this happens.
hopefully it's not the ones that come with the phone?