Loud speaker on my HTC One turns on almost every time I answer an in coming call
This is certainly not the default behavior of the M7 or M8; I've used both.

The way I see it, it's one of the following:


Some option in HTC's phone app or system settings has been enabled to invoke this behavior. I'd suggest reviewing the settings to confirm this isn't the case.
You may be experiencing some troubles with your touch screen. It's possible the touch digitizer is registering false positives, effectively tapping areas of the screen without you doing so.
You may want to check that touches are registering correctly by using an app similar to Yet Another MultiTouch Test and selecting the Paint Tester option. If touches are registered without actually touching the screen, this may explain the odd behavior.


I hope it's not the latter!