For the most part sugar gliders are fairly quiet animals, although they do have the ability to vocalize. Most people are familiar with sugar gliders at different periods when they may be scared, stressed, or nervous and at those particular times they may vocalize. Sugar gliders have a raspy sort of barking type of sound that they will repeatedly make at times of anxiety, times of fear, or at situations where they're uncomfortable with their surroundings.
Most often a sugar glider that is awakened from sleeping during the day, that is startled or unexpectedly picked up or handled may exhibit a serious of these barking type vocalizations that are generally associated with stress. In many cases well socialized sugar gliders that are used to being handled, like Moe here, generally have no reason to vocalize and they don't generally have a need to have to exhibit a lot of sound.
Most often sugar gliders that are brought in to the veterinarian may exhibit certain vocalizations when they feel nervous, when they're around strange people, or when they're being handled in such a way that may be insecure to them.
But overall sugar gliders tend to be fairly quiet. They'll have sort of a raspy, sort of a growling barking sound that they may make at certain times of fear or stress, but for the most part, concerns about them being loud or noisy should not be present, as they're not considered to be a noisy or obstructive type of pet.