Hi!
Please take a deep breath. The infection is sitting in the airways and not in the lungs (that would be pneumonia). Think of it as a piggy version of a bad bronchitis or throat infection (crackling) and a somewhat congested nose (hooting/honking). Since guinea pigs have very narrow and small airways and are not really mouth breathers, every little obstruction is always very audible; sometimes rather alarmingly so.
Please give a pinch of bisolvon powder either syringed mixed with 1 ml of water or sprinkled on a bit of favourite veg twice daily. I prefer the syringe method after some of my own piggies eating everything around the powder, but no medication. Bisolvon is a mucus-thinning product, which will help to free up any congested airways and ease the breathing. Your vet is going exactly the right thing.
You can try and see whether bowl of steaming water (with or without 2-3 drops of olbas oil) next to the cage helps ease the breathing. If it does, then you can continue to change hot water regularly to keep the air around the cage moist every few hours.
Just hang on in there. Switch from the life-long once weekly health monitoring weigh-in and body check to weighing daily at the same time and step in with syringe feeding top if needed. Baytril can in some cases also impact on the gut microbiome and dampen or even kill the appetite. That is why you have got the fibreplex. Most piggies don't like the taste of it, but if you mix the daily dose in a little mushed pellets, it will go down just fine.
Complete Syringe Feeding Guide
Syringe Training Before The Need For Medicating
Who is the boss - your guinea pig or you? ('piggy whispering' tips also work with getting down bad tasting meds like baytril. I prefer to give it in one fell swoop (the unmixed dose is less than a mouthful) and then follow up with water or a piece of cucumber to wash away the bad taste with much fussing over and plenty of praise or you can mix the baytril with the same amount of ribena and get that down in two lots. Either way works and is down to personal choice.
Here is how I control an uncooperative piggy with one hand against my upper body to leave the other hand free for the syringe.
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I hope that this helps?