I created a UX so road warning can be configured using tone, audio or TTS for NextGen9.18.27.687519-Gift(26Jan2017) at [Only registered and activated users can see links. ]
So I know .wav works under NextGen.

You're using WinCE so UX won't help you because the it was for android but unlikely NextGen engine would handle .wav any differently. What is full version of proggy you're using and are you using any skin?