iPhone 6 mute unmute itself

If you are now discovering that after restoring it to factory settings, set it up as new, without restoring your data from a backup, the issue is still present, then yes, it could be a hardware failure.

But we can't diagnose it remotely, sorry.

Have your iPhone inpsected at your most convenient Apple Store or Authorized Service Provider, and they will give you all the available options: Apple - Find Locations - Location Selector

Regards

Giulio

It sounds like your backup's data is corrupted.

No other help for you, except adding apps, data, settings, once at a time.

As far as contacts, messages, photos, and so on... you can sync them to iCloud, then retrieve them as soon as you enter your Apple ID credentials.

Regards

Giulio