They're called Laryngeals, and no one really knows how to pronounce them, from what I can tell.

Edit, there are two theories on how to pronounce them:

Rasmussen chose a consonantal realization for *h₃ as a voiced labialized velar fricative [ɣʷ], with a syllabic allophone [ɵ], i.e. a close-mid central rounded vowel. Kümmel instead suggests [ʁ].