It's terrible... I heard 'let's turn you into a unicorn' and 'let's not tell grandma'.
The best thing is for everyone to contact them.
I have to add I'm finding this a common problem, not only with ads but programmes too...I've no problems with accents, they just don't seem to speak clearly enough these days.
The producers would probably argue that the actual words of the conversations are not important - they are just showing the spirit/joy of Christmas.
Or they may say they have done this on purpose to get people talking about the ad like we are!
I'd not call it mumbling, I just think the sound engineer got it wrong and muffled a lot. (Although I think I can catch more than some here are claiming.)
When I saw the advert last week I said to my OH that I couldn't hear what most of the people were saying - I saw the ad last night and the voices were very clear so the sound has been tweaked.