Cortana has lost her voice on my HTC One M8. She responds to everything with text-to-speech. Even if you ask her to sing, or tell a joke - it's all text-to-speech now.
Just out of curiosity, did you turn location services off, either purposely or accidentally? The settings can be found in settings > location. My coworker had a problem with Cortana that sounds very similar to yours and he had inadvertently turned location services off, which disables Cortana.
Nope that's not it. I even turned off Cortana - restarted - and set her back up. Same thing. She just "sang" frere jacques and it was disturbing.. The text-to-speech engine couldn't pronounce the words right, so it sounded horrible.
I have the same issue every has worked fine then last night she has only text to speech I can't work it out and resetting my phone isn't a real good solution