This issue also affects users of CyanogenMod 11 which is currently based on Android 4.4.4 (https://jira.cyanogenmod.org/browse/CYAN-5728). The same thing happened to the HTC One M7, but HTC fixed it quickly with an OTA update (http://venomroms.com/venomhub-fix-no-sound-calls-htc-one-m7). All in all it looks like the malfunction is caused by an automatic update of the Google Play Services framework. Not reacting at all so far is definitively not a good move of Google.
if it's a bug in a closed source Google service, how HTC released a fix? even if they roll back the service on a new system image it will self update... unless they hacked away and bumped the version...
The services probably just trigger an error deeper in the call stack like in Qualcomm's proprietary binary blobs. Perhaps they received a new version from Qualcomm.