I have two border collies who have lived outside all of their lives. They only come in at bonfire and at new year when there are fireworks.

The oldest is 14 and 8 months and the younger one will turn 13 on the 21st of this month. They are mother and daughter.

The older one, Mist, is going a bit deaf these days but can still hear you speak to her when you are relatively close and can obviously still hear fireworks and thunder etc...
In the last few months, she has started getting quite vocal, whinging and whining often during the day and barking at night. She isn't very loud and does it from inside the kennel but I don't want it to start being an issue. My brother and his GF and their kids next door haven't heard her at all but I am losing sleep over it and I worry other people might hear.

At her age, I know she's not going to be around that much longer. Apart from arthritis, she is ok but the arthritis will get the better of her soon no doubt and anything else could happen suddenly as we all know.
I have a plastic box muzzle like what greyhounds wear in their kennels and I found that putting one of these on Mist when she barks helps to shut her up without stopping her from drinking and all the rest but I don't like doing it and I'm also not sure how long the effect it has will last.
The other dog is fine and they are kept together and she's always been fine before so why start now at this age?
I can't bring her in because the parents won't allow full stop and I can't afford to move out. I can't rehome her at this age. She wouldn't cope with somebody else now and it's just too long to have owned her. 14 and a half years is a long time. I'll need a month off work when she dies.

I take them for a walk 2 or 3 times daily and the last one is as late as I can get it, about 10 pm, but I still find her start barking in the early hours. It's only since December that I've heard her barking at night and she doesn't do it everynight. In fact, there are more nights she doesn't than when she does but she never used to do it at all.

Any suggestions would be appreciated.