sounds like your volume down button is stuck. See if you can see something blocking it and try to remove it if you see somehting. If not, follow this guide iPhone 3GS Volume Button Replacement to get to the switch and see if you need to clean it. You really only need to get to step 21 where you can access the electronic switch and take a physical look at it. If there is something blocking the switch cover itself from going back up, then you have to go all the way and possibly replacing hte cover. If physically damaged you might need a new volume cover, which you can get here iPhone 3G and 3GS Volume Button
I spent several frustrating hours trying to find the cause of this annoying problem that recently started to bug my iPhone 4S. Then someone in this forum mentioned about the volume control button getting stuck because something is blocking it. In my situation it turned out to be the protective case (LifeProof), so all I had to do was to reopen the case and reinstall it properly. Voila! A simple fix for what actually was a simple problem!