Try GoPro CineForm Studio
I found the free GoPro Cineform Studio works great to view the GoPro Videos.
WMP just played audio, like the OP's problem. But other viewers had trouble keeping up with the 1440 48 FPS mode on a medium high quality computer. There was a lot of frame skipping.
Cineform displayed nicely, and has a full screen mode that played nicely without frame skips on the 1440 48 FPS mode.
Cineform also allows conversion to AVI or MOV with some intial special effects. It's a good powerful video pre-processor (contrast, framing,...) before export to other editors.