Very helpful I must say
Erm, it definitely is useful to get a feel for how many others are experiencing this. Unlike the 8.0.1 issues on iPhone 6/Plus, where everyone piled in to say they also lost service and Touch ID, only two people including OP have this issue in this thread.
OP, I have an iPhone 5 and haven't seen this. dannyyankou's advice was perfectly valid. You've tried the factory reset, now take it to an Apple store or go to the Apple website to log a support issue.
edit: Just thought something you could try. Try enabling or disabling the 'Change with volume buttons' in Settings > Sound