DuMaurier's Son Faults Harris, Director
I can't link the article, but there is an article in the Radio Times interviewing DuMaurier's son.
He said Frost's script was wonderful but the sound issue ruined it for him.
More to the point, he came right out and blamed three people: the sound guy (well, guys/girls I assume), the director and Sean Harris. Mainly I think he faults the director for LETTING Harris mutter and the sound people for not speaking up and/or fixing it.
He also said that it was a relief when Joss died so at least the final part of the series wasn't ruined.
OUCH. That is HARSH!