Guinea pigs breathing should not be silent and you shouldn't be able to really see them breathing, either.
One of my girls, Dolly, is sometimes very audible in her breathing, almost sounding like theres a clog in her nose. (vet says its in the lungs) My vet says it would be more uncomfortable for Dolly if we treated it, and as she's not doing it recently anymore, theres not anything we can/or should do.
If it sounds more like sniffling, like a cold, for your girl, I'd say to keep a close eye on her to see if there is discharge coming out of the nose/eyes, because then it would be important to get her treated. My other girl Pigeon did have this happen to her, all the while I had my window open and fan on in the room that the pigs are in for days, and after I closed the window, turned off the fan, after a couple days Pigeon was back to normal, so perhaps whatever room your pigs are in are too drafty?