In that case, you definitely want to select “Audio”. Because of how we operate the PLL in the Bartók, there shouldn’t be any noticeable difference in sound quality comparing Audio to Master, and Word Clock would only be relevant if you were using a Master Clock.
If you don’t hear any irregularities when using “Master” like clicks or dropouts in the sound, then it means that your CD player’s clock is running at a very close frequency to the Bartók’s clock, so the sound never has to stop and re-lock. It’s still a good idea to run the Bartók in “Audio” mode, just to be on the safe side to ensure you never get clicks or dropouts if the clock frequency of the CD transport or Bartók happens to vary for some reason.