Long story short.
I am on Sprint. I have a launch day silver/white htc one.
Use the front speakers at least 2-3 days a week watching YouTube and the occasional blinkfeed video.
no speaker issues at all.
my point?
massed produced products can have issues with build quality as it seems you are experiencing.
so, either return this 3rd one and go to another att store that may have a different batch of units, or simply return the phone as it has failed your expectations.
Perhaps writing a letter to htc and expressing your experience with their products may help them in the future with their product assembly lines.
coming on a website that is not official to htc and asking users "what is going on" will not merit much fruit i am afraid...
if you are that upset and let down (not me as the phone has exceeded my expectations), speak loudly with your money. Walk away and pick up another phone with another manufacturer.