UPDATE

Spoke to Sony Canada Customer Care yesterday (I'm only here on the Sony Ireland site as this was one of the results that came up when I Googled the issue), and the person I spoke to said that they had received several reports of this issue so were aware of it. The person I spoke to said that their engineers were now determining if they were looking at an issue concerning a batch of faulty units, or a larger design oversight/problem.

They offered to replace my headphones with another unit which I held off on for now as I'm fairly sure it's a broader design problem, although I may give it a shot in a week or two just in case. If it is determined to be a design issue with this model then Sony will notify customers through online channels and offer a fix of some sort be that a firmware update or a redesigned replacement unit (XM3.1) or whatever - this should be available to all original owners of the XM3s regardless of whether their warranties have expired (I was concerned about this point as mine are replacements so only have a 90 day warranty).

In the meantime, the only way I've found to use the headphones in the cold is wired. I already had a lightning to 3.5mm cable that lived in my bag for those occasions where the headphones run out of juice, so now I just use that when I'm outside (the headphones turn on automatically when you plug in the cable, so you need to be sure to turn them back off otherwise the mute problem remains). This is obviously far from ideal - the active noise cancelling and bluetooth were the key reasons for the purchase after all - but at least I have music (and warm ears) for my commute. The design means they offer a certain amount of passive noise cancelling anyway.

Finally, I just urge everyone who is experiencing this issue to notify Sony Customer Care wherever they are to make sure they are aware of this problem and hopefully speed up the resolution.