WECv7 is just an evolution of "Windows for Embedded devices." Originally called PocketPC, then WinCE, now WECv7. Different tune of same song. The layers beneath application is irrelevant for playing audio. If you hear voice direction, then NNG codec for .ogg is working. Unless NNG specifically made a version just for WECv7 (which I doubt), it should also has codec for .wav and therefore should be able to play .wav files. This is assumed that configuration is not preventing it. Have you tried to contact dealership and asked them why alerts are only in "beep?"
TTS is more complicated because you must have 3 things:
1. Library (.dll's)
2. Voice data (how it's going to pronounced)
3. Voice configuration file (so it's available to proggy)
So far I never seen any TTS library for WECv7 anywhere much less data or config.