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JAZZ Torme (two discs): ''Encore at Marty's.'' Torme, vocals; Mike Renzie, piano; Jay Leonhart, bass; Donny Osborne, drums. (Flair/MCA PG8200.) Also, ''An Evening With Mel Torme and George Shearing.'' Torme, vocals; George Shearing, piano; Brian Torff, bass. (Concord Jazz CJ-190.) Torme is riding high these days, with more success than he has ever experienced over the many years of his career. And it's no wonder, for he sounds better than ever, and there seems to be no end to his inventiveness - he can do just about anything with his voice. These two latest from Mel show how important environment and backup can be to a singer. Although both albums are excellent, Torme seems more at home on the Marty's set - perhaps because of Renzie, Leonhart, and Osborne. Although Shearing is one of the greatest jazz pianists alive, and has been touring with Torme and Mulligan regularly, one never feels that he is really an accompanist at heart, and there are times when his playing behind Torme actually becomes distracting. Bassist Brian Torff, a technical wizard, has unfortunately become somewhat of a parody of himself. His tone sounds whiney, and his penchant for playing as many notes as possible has gotten to be a bit much. Nevertheless, the Shearing/Torme sides are eminently listenable, especially Torme's superb rendition of ''A Nightingale Sang in Berkeley Square,'' and Shearing's touching introductory vocal chorus on his composition ''Lullabye of Birdland.'' Torff and Shearing do a bass-slapping, rollicking duet on Torff's ''Manhattan Hoedown.'' The Marty's set is even better. Torme is as polished and slick as a bowling alley floor, and proves once again that jazz and cabaret can mix. He cooks and swings his way through a series of well and lesser-known standards (including Harold Arlen's lovely ''When the Sun Comes Out'' and Cole Porter's ''Looking at You''), and a charming tribute to Fred Astaire -- a medley of many of the great songs Astaire he introduced in his movies.