I don't think my dark brahma roo is more quiet than other breeds, he just doesn't have alot to crow about.
He crows a few times in the morning. He crows when they get an early crowing roo in the commercial house down the way. He crows big time when we have cars drive up, which is rarely. He also crows when I hide and play my rooster crowing ringtone, which I do on occasion just cuz I like to mess with his head.
Now the egg song is a different story. He sings it louder and longer all day long, as the hens are singing it.
I think alot depends on their enviroment. Our farm is very isolated. Very little traffic in and out, no light at all at night (pitch black). No other roosters around to compete with (unless, like I said, they get a roo in the commercial houses about 1/4 mile away).
In town or suburbia he might crow non-stop. Hard to tell with these roos.
To add to that, my Brahma hens seem to have lower, coarser, quieter voices than the rest of the hens. More like a duck than a chicken. If it held true with roosters, I can't imagine them making really good full on crow.
Funny I came across this question right now. It's 5:30 a.m. here and barely light ... and I was just thinking I can't believe my neighbors don't get mad at our Brahma rooster for crowing non-stop at this hour. He's been going at it for 15 minutes. And I think plenty loud ... the windows are closed and he is not very near. It's a gravel-ly crow, but nonetheless ... That being said, I LOVE my Brahma rooster, he is handsome, gentle , and easy on the girls