I still continue to have this issue. I fix this by going to settings> general> accessibility > AssistiveTouch.
Turn AssistiveTouch on, you should see a white circle on upper left corner, tap it to see a pop up of four icons. Tap the device icon,another screen appears where you seen an bell that you can unmute/mute. That fixes mine every time. Don't know why it mutes itself here even though nowhere else on my phone shows the phone to be on mute!
Hope that helps,