Zone of Silence
The Zone of Silence was an interdimensional void discovered by Warlord Kodos and used by Doctor Robotnik as a prison. A number of individuals were exiled to the Zone, including Ixis Naugus, Kodos himself, an entity named Feist, and King Maximillian Acorn.
The Zone was home to a race of imp-like creatures, and apparently had detrimental effects on those imprisoned that could only be resisted by a crystallization process invented by Naugus. All the prisoners (save Feist) have since been released and converted back to normal.
The Zone of Silence originated in the Saturday Morning canon as the Void. Some accounts of it have referred to it as being literally devoid of sound, but most ignore that.