Yes, an audio glitch probably, but "Dewis" is pronounced as you would expect.
the audio definitely says stewis. I listened to it several times and had no idea it was "dewis"
The course teams cannot fix audio issues. All we can do is to delete the sound completely if it is very badly wrong. In this case it is just a minor glitch, and very good pronunciation guides are available in the Welshplus pronunciation videos that we recommend at several places in the course notes. There is very little variation in the pronunciation of consonants in Welsh.