Yeah, according to some internet search, the factory reset does not seem to have helped anybody. But it still may help you, who knows. If you give it for repair, they will factory reset it anyways.
You can also try safe mode. It is very similar to a factory reset. No downloaded app runs. But it is temporary. After a reboot, you are all fine.
Make sure "Fast boot" is not enabled,
Shut down your phone,
When you see the HTC logo, press and hold volume down button.
It will open in safe mode. Not a factory reset but if an app causes this problem it will reveal it. The only difference is, the settings are saved and the only settings which are relevant would be volume itself and beats audio, so it shall not differ between a factory reset and safe mode.
By the way, HTC One's speakers are loud, but not extremely as thought.
For example, Note II and Nokia 920 probably have louder speakers. But HTC One's speakers are much more clear, and basses are so vibrant.
So its speakers are not the loudest, but the highest quality.
Low volume might be due to the file you are playing. For example, flac files are usually very silent with respect to mp3's.
But still if you think your speakers are really silent, maybe due to the update or so, you may try safe mode, then a factory reset, or even get it repaired.