I agree. I can't get "bia" out of what I'm hearing. I couldn't figure out what "dia" would mean in the context of the exercise but I put it anyway. I've reported that the audio does not sound correct.
[As I remember, it used to be that, when you submitted a report, you could add explanatory text. Now that facility seems to be gone.]
Out of interest, I listened to the pronunciations of "bia" on the "Foghraíocht" section of Teanglann.
While the "b" of the Ulster and Munster versions seems quite clear, the Connaught version sounds, to me, less distinct: sometimes I can hear it as a "b", but very often I hear it as a "d", as in the audio on this exercise.
Perhaps in the context of a full phrase or sentence, it would be more readily perceived (or
I would perceive it) as a "b".