It's tough to give a pat answer. Situations are different, wind, cover, what proximity you think thier at, will affect how loud you need to be.
I use hand calls and if I'm not in wide open country I start out about mid volume and try to sound as real as possible. If you have seen one or know they're close squeeking lips works, or a sqweeker. It doesn't take much to get them interested and to much sqweeking seems to make them cautious.
I like to "blow them a kiss" when they're trotting and I want to stop them.

I agree with BigNate. Situations differ. Just try to sound natural. If you've ever cleaned a rabbit, you know they're lungs don't have that much capacity, so the way I try it when first calling is short bursts 1 to 2 minutes apiece and wait a few minutes. Volume doesn't matter to me as much a timing to me. We called two yoteys in the other night with about 3 minutes of howls and yipes. As soon as they started to respond with they're own vocals, we shut the call off, it worked pretty good. Just experiment and you'll get the hang of it. Happy hunting.

Whenever I've heard a rabbit caught by a cat squeal, it wasn't that loud.

I agree with what everyone has said. I hunt in an area that is very big hills and draws with open pasture and timber. Sometimes do to wind currents I will abandon the soft call for a louder call just to get their attention. Then I go back to soft calling. A pattern that has been very good for me is to call soft and wait several minutes and then medium to loud depending on wind noise and terrain. I then always go back to soft. Becasue of heavy cover I have found that calling louder and louder spookes the animals I didn't see come in. As to how far a coyote can hear it is unbleivable.
Studies on coyote's say they can zero in to with in several feet of the caller from 2 miles away. I have had 20 mile an hour wind blowing into me and using a range finder used a small yote squeaker and had coyotes hear it from 359 yards away. I have killed 20 coyotes called in this year and 10 of those where pulled in with the yote squeaker. I am a believer that once you see the coyote put the big calls down.Just my 2 cents