They make several distinct sounds.
Barking: One is a "bark" that is similar to a puppy yipping. The precise reason sugar gliders make this sound is unknown, but it may be in response to being frightened or to call out to other sugar gliders.
Purring: Gliders also produce a "purring" sound when they are content, but it is very faint and hard to hear.
Crabbing: Sugar gliders also make a "crabbing" sound, which is a defense mechanism and indicates fear. It is been described as sounding like locust at night with an escalating, then decreasing pitch.
Hissing: "Hissing" is similar to the "crabbing" sound but comes in more short and deliberate bursts. It is thought sugar gliders hiss to indicate annoyance.
Chattering: "Chattering" is similar to squirrel chatter with very short, quick taps. It seems to indicate excitement.
Crying: Sugar gliders also have a "crying" sound that is often associated with separation from cage mates or family.
Singing: Sugar glider females with young are said to "sing" while the babies are still in the pouch. It is described as a soft, rhythmic sound that changes in pitch.
Scent-Marking: Scent-marking is a natural behavior for sugar gliders. In the wild, dominant adult male sugar gliders scent-mark their territory and other colony members. In captivity, male sugar gliders mark other cage members by rubbing their head and chest glands on them.
They also scent mark objects in their cage. In general, sugar gliders do not emit an offensive odor. Mature males of breeding age have a slightly stronger odor than females.
Sugar gliders use body language to communicate mood. Sugar gliders can take on a defensive position by standing on the hind legs with head extended forward and mouth open. Or, a sugar glider may lie on its back, vocalizing with all four feet extended toward the perceived threat. If a sugar glider takes these positions when you attempt to pick it up, try offering the flat heel of your hand for the sugar glider to smell until it calms down. If the sugar glider bites you on this part of your hand, the bite shouldn't be deep.
Signs of chronic stress are irritability, not wanting to be handled, more time in nest than playing, poor appetite, self mutilation and hair loss.
Full credit of "Sugar Glider Communication Revealed By Audrey Pavia" goes to -Small Animal Channel-