I've done this using Google Glass and it works well. I'm the developer of BASEline Flight Computer, so it was fairly easy to port the Android app to Google Glass. You can find used Glass on ebay for around $400.
As you mentioned, the advantage of a HUD is that you can see horizontal speed, vertical speed, total speed, and glide ratio all at once, and with much faster update than you could ever get with audio. I personally chose to display your speed as a "polar chart". Horizontal and vertical speeds on the axes, total speed equals the hypotenuse, and glide is shown by the slope:
To get good data, pair the phone with a bluetooth GPS like the XGPS160.
Let me know if I can help with anything. I would love for this to exist! But I don't expect most people to deal with google glass. If interested, the google glass version of BASEline is available on a github branch: https://github.com/platypii/BASElineFlightComputer/tree/glass