Voice Locks are a security measure found in Glukkon factories. The locks require either a Slig or a Glukkon to repeat a voice pattern before unlocking. Sligs can not use a Glukkon voice lock and vice versa.
In the case of a Slig voice lock, the lock will automatically greet anyone walking past it, including Mudokons, with a simple 'hi'. When a Slig replies by saying 'hi' back, the lock will produce a series of sounds. After this the Slig will have to repeat the sounds in the right order to unlock the lock.
The voice locks for Glukkons are somewhat easier and do not require the repetition of a series of sounds. Instead the lock simply needs to be told 'do it' after the initial greeting to unlock.