They're clearly not bothered about it.
That's a bit of an assumption to make - unless you know what the fault is and how easy it would be to fix. It may be that they have no means of sorting the problem prior to returning to their normal studios.
Well noggin, like most people on here, all I can do is guess. For what it's worth, I don't think it's a terribly unfair assumption.
The fact of the matter is, that sound quality on the News Channel and BBC One bulletins has been poor for over a week. I listened to a BBC One news programme last night and during many of the reports, it sounded as if the speaker on my TV was knackered.
I would like to think that some sort of testing was carried out prior to the move to N9? Assuming that such testing was performed, then I can only surmise that someone decided that the sound quality was of an acceptable broadcast standard OR that it wasn't worth the money to fix it and they could probably get away with it, as this setup would only be in place for a short period.
As a viewer, I frankly don't give a toss about "what the fault is" or "how easy it is to fix". That's for the engineers. The fact of the matter is, the sound quality is unacceptable. When people I know who are not interested in such things start to complain about the dodgy sound quality, then I think we have a problem worth sorting.