My guess is just that. A hole is needed for a microphone when recording video on the rear camera.
Also, a more granular forum might be http://www.howardforums.com/forumdisplay.php/157-HTC
Hello again, I have a new HTC ONE M9 phone and I have several questions about it, if I may. Thank you, R. Proffitt for the link review to this phone. I can't find any info in my travels on this question. On the back of the phone, at the top, there is a pinhole opening within a thin gray line. This thin line runs horizontal across the entire back of phone. The gray line on the back of phone 'T's in the middle of the phone and the line runs a short vertical to the camera eye. What does this pinhole do or is used for? I took a screenshot of the review and cropped it down. Hoping I can add it for reference. I have other questions and will ask more again within this thread. Thank you for all your great help here!