I agree with Cathie, I tried everything, restarting it, wiping it, the lot. My phone would not show the volume bars and I would hear no sound when playing something.
From what I can gather the phone is thinking it is docked i.e. to a speaker of some kind and is getting confused. There is a sensor in the bottom of the dock which is supposed to sense if anything is connected. However if dust allows to gather at the bottom then this is where the issue starts to creep in. I had used a toothbrush before to clean the gap but when I used a toothpick I found there were large clumps of dust and fluff that came out, and hey presto it worked. This type of stuff isn't just you're loose fluff that blows around, this is stuff that might have been in there a long time and started clumping so when you clean it, make sure something comes out.