It's basically dirt built up. If its driving you crazy and you don't have ear buds present or the alcohol. Then follow Sally's advice it worked for me.
" go into Settings, General, Accessibility, AssistiveTouch, and turn it to ON. This brings a little white dot onto the main screen which can be used instead. Then tap on the white dot, go into Device, then make sure the Mute/Unmute says Mute (as in, you have to press it to Mute it - if it says Unmute it means it is muted!!). You can also adjust the volume settings from this spot. I hope this helps someone!"
Great advice. Easy and WILL work :)