The first sound you'll hear from a sugar glider is quite startling, considering their small size and appearance. I think it was Tim Hussey who first described it as crabbing, which is really an apt description. My kids say it sounds like an electric pencil sharpener, and that's about as close a description as I can come up with. (Thanks to Robbie G. for the .WAV file!)
They also make a barking sound , which sounds just like a very young puppy barking. I discovered this at 3:00 am the morning after I got my first gliders. We DID have puppies at the time, but they were only 2 weeks old, and even in my groggy sleep-ridden condition, I knew chances were pretty good it wasn't them doing the barking! Sure enough, it was the gliders. There seemed to be a definite cadence to the barking, and only one of them barked at a time. If that one stopped, the other one would take up the cadence and continue without ever missing a beat. If you live in a small apartment and the noise is a problem, leaving a small light on in the next room seems to help keep the barking to a minimum, although I'm not really sure why. I think they use this sound to try and locate other gliders; from the mail I get it seems that the barking is much worse when they are kept alone.
One thing I can't stress enough, so you will probably see it repeated often in these pages, is that gliders should NEVER be kept singly. They just don't handle it well, because they are community dwellers and need the companionship of others of their kind to the point that they will actually die without it. If you are planning to get only one to save money, don't. You'll end up buying three because the first one will die and you'll know you have to buy two the next time. So it's better to just wait until you can get two, and save yourself the grief and expense of losing the first one!
When they get annoyed at each other, they make chattering sounds that are very difficult to describe. They have a lot of "S" sounds in them, and sometimes it really sounds like they're cussing each other out. Other times it almost sounds like sneezing. When our second male arrived, the female had already been here for a few weeks, and he was very uncomfortable about her insistence on sniffing every inch of his fur. He sounded like he was saying "shook 'em, shook 'em!" in a loud whisper.
They also make a variety of quiet chirping / chattering sounds; you'll usually hear those coming out of the nest box as they settle in to sleep or shift around from time to time.