I hate reviving old threads but I am encountering this now on my Verizon lollipop M8. I turned off the Google voice detection on all screens to no avail. Then I find a nearly identical setting in Nova Launcher settings. Sadly that made no difference either , but good to know if that is the cause for others.
I narrowed the problem down to AudioManager Pro. The moment I uninstall it the camera works great. I hope they update AM Pro soon but I doubt it. It hasn't had an update since 2013.