Hi Annie, we have lots of cats around here too. I have 2 neutered females, one of them is upset to the point of her spraying upstairs.....so she's banned from upstairs.
The other one was scratching furniture more and more. I got some Feliway plug ins and that seems to have helped a lot.
The tom who wanders round here does a fair bit of yowling, I think it might be to voice a territory as well as scent marking.....especially if new cats arrive in the neighbourhood.
BTW my first two points on my post is due to this tom 'claiming' our garden as his territory. Which is why I got Feliway.
I appear to be babbling.......its late and my head wants to shut down but my body wont let it lol. I hope some of it has made sense.