Encompass New Opera Theatre / Léirmheasanna

George Antheil's satirical one-act opera ''Venus in Africa'' is set in an outdoor hotel cafe in North Africa. And that's where the cast and audience must have felt they were when Encompass Music Theater gave the New York premiere of the work on a double-bill with Britten's dramatic cantata ''Phaedra.'' Encompass is a scrappy company that presents mostly contemporary and always unusual musical theater works in the Salon Theater, a loft on East 91st Street near First Avenue. The few amenities for patrons include an air-conditioning system that doesn't do much. So it seemed that not bright white stage lights but the African sun itself was bearing down on the perspiring singers and the wilting audience.