"He likes to hear his own voice."
Translation:Han tycker om att höra sin egen röst.
It’s not that simple, normally you put adjectives in the definite after possessives (which is identical to the plural), so you say for example: ”sin stulna bil” and not ”sin stulen bil”, yet you say ”sin egen” because this is an exception to the rule. Nevertheless, many people say ”sin egna” colloquially. Read more about it here.