T-MOBILE here. My speakerphone calls & normal calls are OK, w/an occasional rattling/metallic sound on normal calls, as if a speaker is damaged. But, that's not the case, as audio w/media is fine.
BLUETOOTH however, is a different story, definitely something going on there, as callers on the other end make quick note of the problem. (Used 2 different B/T's, same results).
Possibly a network issue as well,I'll go that route next & contact T-MOBILE to see if others are experiencing issues in my area &/or if it's an HTC ONE/T-MOBILE specific (known?) issue.
FWIW,my phone is less than 3 weeks old,manufactured on 4/16/2013.That lends to me theorizing the issue is software &/or network issues & not a hardware defect/manufacturing problem, as most, if any issues of that nature were probably addressed beforehand.