Due to a bug in the Windows audio interface, in rare circumstances the audio output device may become unselected by Windows, thus leaving no default audio output device. This error can be seen in the log.txt file by:
OpenGPWS[xplane.c:285]: OpenGPWS enabling
OpenGPWS[wav.c:224]: Cannot init audio system: device open failed.
OpenGPWS[wav.c:608]: assertion "alc != ((void *)0)" failed:
This can be easily fixed by going into Windows settings, and setting your default audio device. If a device is already set, try changing it to a different option then setting it back to your desired output.