Yes, it is, and the only time they make any noise like that is when they're sick.
I also had it, and still do, in my stock. For months I have kept them seperate from any new mice, treated them all with Baytril and then a cat antibiotic. Having seen no signs in my mice for two months, I put a new mouse in with one who had it... lo and behold, the new mouse now is sniffling. Its a nightmare, and I underestimated it.
They CAN recover. Mostly they develop lung scarring, and then it can reoccur. With pets its possible to help them live comfortably for some months, but with breeding stock there seems to be little point.
None of my previously infected does passed it on to their offspring though, interestingly. I kept the babies for a couple weeks extra and sold them on to people who knew my circumstances and warned them to quarantine and check them, and there have been no problems reported. This could just be incredible luck, however, and I won't be breeding anyone who's shown signs of infection again.