[One (M7) Q&A, Help & Troubleshooting] gustav30:
I did exactly that! I'm on Viper 3.5 stereo works great!

I did some extensive testing over the weekend on KitKat - what makes it breaks it with a possible work around. Here are some tests and results:

First are the tests that break it:

Open the camera app take first video. While staying in camera app, take 2nd video and or 3rd.

Results:

2nd and 3rd video will not have stereo, only left-channel audio, only first video will have both channels for stereo.

(Only difference here is closing camera app and going back in.)

Open the camera app take first video. Close camera app and go back in to take 2nd or 3rd video.

Results:

2nd and 3rd video will not have stereo, only left-channel audio, only first video will have both channels for stereo.

Now, I previous thought only a reboot will enable stereo recording again, but I found a temporary fix:

Now for a temporary fix:

After you take your first video that will be in stereo, exit camera app, and open Google now. Just click the Google voice button. Doesn't matter if you say anything. Now go back to camera app and take 2nd video. The second video will now be in stereo without having to reboot phone!

After taking 2nd video - go back to Google now and open Google voice again. Again doesn't matter if you say anything. Go back to camera app and take 3rd video - it will be in stereo.

So for some reason Google voice resets whatever gets broke by the camera app for the 2nd microphone.

This method is kind of a hack job, because you have to remember before taking a video to open Google Voice if you want stereo recording. This can be impractical if you are with friends/enjoying a concert to remember after each video to go to Google Voice.

Also - this bug is present with Zoe from the camera app......and this is what killed it for me to go back to a 4.3 Jellybean rom:

Lets say you open the camera app and take a Zoe and want to take a video right after the Zoe....The Zoe will be in stereo - but your video will not be, only left-channel. So...even after taking a Zoe, you have to go back to Google Voice and then go back in to Camera app and take video.

This was the killer for me. Too many steps to remember. KitKat not worth staying on as I'm constantly taking Zoes/videos of my 6 year old. Can't be going back to Google voice.

I opened a ticket with HTC and it has moved its way up to tier 2 support. But I'm not holding my breath on it.

If you are on a KitKat Google play edition Rom this bug is avoided, since that camera app only records true Mono, so if you want to settle for mono you might be able to grab that camera from the play store.

If anyone has any suggestions it would be most welcome! I've thought of copying older jellybean camera libs to KitKat but don't have the time to test and see if that will make it or break it. Also I've tried some of the Sense 6 ports - and it seems the microphones are really sensitive in the camcorder app where a lot of clipping happens. Reminds me of my old Sensation, the way Sense 6 records video on the One. (I know its a port).

So for me right now Viper 3.5 is the way to go if you depend on stereo recording and can't be bothered with the google voice work around.

Hope this helps!