Al Jarreau's tinny tenor, loose vibrato, and percussive scat have always borne odd charms on Accentuate the Positive. But for years he's buried himself in smooth R&B mixes. This welcome return to jazz is mixed. Jarreau glides giddily through Dizzy Gillespie's ''Groovin' High'' and swings urgently on Eddie Harris' ''Cold Duck.'' But a heavy backbeat obscures Lionel Hampton's ''Midnight Sun,'' and Jarreau's falsetto falters on the ballads.