If it's respiratory illness, I *think* you would be seeing more symptoms by now, like runny nose, teary eyes, decreased appetite and activity. But the Vet Rx poultry remedy is for respiratory issues as well as other things. They *can* have issues, but they're far less susceptible to illness than chickens. When I've noticed sneezing, it's bc they've got dust packed in their nostrils. I use a qtip to clear that out for them at times.
You didn't mention flock dynamics, age, how many you have, etc., so I'll describe life here & you can see if it fits, bc this is when I notice the hoarseness in the girls voices:
it's mating season, so Mama Rose has gone from murmuring "cocoa" most of the time with occasional alerts to shrieking nonstop for her mate to "Comeback! Comeback!" (Most describe this as "buckwheat", but I swear they're jealous hennies afraid their mate has a wandering eye).
She does it in the coop, out of the coop. I don't let her roam free during mating season bc I learned my 1st yr that's a good way to lose 1/2 your flock. If the ladies are kept in the yard, the males stay in the yard.
But they do it so much they can become hoarse. After mating season is over, she'll go back to quietly mumbling abt cocoa.
When they are free to roam, they'll wander off to find a hiding place to nest. One male won't leave his mate's side, another won't stay at his mate's side.
The 1st time we went through this, I remember commenting that it was like they were playing house. We were in lockdown and I was fascinated by them, so I wld sit outside and watch them during the day.
It was their 1st yr, there weren't any eggs yet, but Mama Rose wandered off and made her nest on her own, then called out, & her mate came running. At dinner time, they both came in, slept in the coop, returned to the nest, stayed for the day, rinse & repeat.
Her sister's mate didn't stay w/her all day, so she would call out frequently.
When you spend that much time w/them, not only will you be able to identify which bird is calling, but also notice a difference in calls.
Usually it's a checking in signal: I'm here, all's good, you still out there?
Then there's the distress signal: something's wrong, come quick.
I'm pretty sure there's a 3rd one that's basically a complaint that she's bored and expects him to come be w/her, bc remember, I was watching, I could see where she was at,nothing was wrong, but it was a good way to make him come running.
Both Damsels had guards at the nest aside from their mates.I'm not sure if that's normal or just the dynamics of having a small flock.
They had ALL stayed w/the sister, she nested 1st, until Mama went off to build her own, and then they split, one mate,one guard,one hen, per nest. The guard settled in about 3-5' from the nest.
But I'm wandering away from your original question. Hope this helps.