I know this thread is a few months old but I'm having a similar problem so I thought I'd chime in.
I had a flock of 50 (half hens and half roos) but a mink got in and killed all but 7 at about 5 months of age. Only one roo survived and he was defo not an alpha roo.
I hadn't heard him crow at all since the rest of the flock died (about 3 weeks ago). So, a week ago, I went into the coop and started crowing to him. He got all fluffed up, flapped his wings and finally let out a crow even though it was rather a sad crow. Anyhow, since that day, when I go into coop he starts crowing like crazy.
This morning when I went into the coop, he started crowing as usual but I noticed it sounded quite weak. On closer hearing, he sounded as though he is losing his voice. The crow was raspy followed by an airy breath. I'm concerned he may be sick but se no visable problems otherwise. Wonder if he's just strained his voice from all the sudden crowing in the past week? Time will tell.