One way of reducing air borne sound is to line the walls with foam wedges, set at right angles to each other, A super cheap alternative would compressed paper egg boxes, to do the same thing, absorbing and not reflecting sound.
But since the room becomes effectively an anechoic chamber, it can be very disconcerting. You may well hear your breath rasping and your gut gurgling, and your voice does not sound like the one to which you are accustomed.
Years ago, Goodmans, the speaker manufacturers, in Wembley, lost one of their senior sound engineers. He got locked in an anechoic chamber.. Since no one could hear him shouting or knocking he was there all night. In the morning, poor chap, he was a complete nervous wreck.
So maybe do not line all the walls and ceiling!
Howard