re:
Try turning off your Bluetooth. Seems like on the windows phone Bluetooth interferes with wifi calling on T-Mobile and it also stops speech. Perhaps many other things, but this is a known defect.
I was walking on a trail today and received a text message and my nokia 521 started the speech readout process. When I got home and my Bluetooth connected to a device, speech stopped working when I was testing via Google Voice messages.
Turned off Bluetooth, sent a couple of google voice and speech was back on.
By the way, if you have T-Mobile and need wifi calling because your signal stinks at home, T-Mobile will send you an in home cell booster....for free but you must return if you cancel the contract before 2 years. I'm on a month to month now and they still sent for free.
Good luck.
If anyone has a solution for speech with a connected Bluetooth device, but not a earphone, let me know.