If Al Jarreau's quintet ever fails to make it to a concert on time, the singer could easily go it alone. Last night at Wolf Trap Jarreau was a veritable one-man band. Throughout the evening his voice, surely one of the most resourceful in pop music, reproduced the sound of flutes, tin whistles, snare drums, congas, cymbals, saxophones, guitars and the wordless but joyously expressive rhythms of scat.

Unfettered by the commercial restraints imposed on his recent recordings, Jarreau's wondrous baritone soared from one song to the next while his body percolated with rhythm. The only letdown in the show came when he dwelt on songs unworthy of his talent. Happily, that seldom occurred. The high points of his concerts are still his feverish arrangments of Paul Desmond's "Take Five" and Chick Corea's "Spain." The first was an improvi-sational tour de force and the second perhaps more than any other tune, revealed how Jarreau can remain true to a lyric despite his awesome technique.

A local band, the Pete Kennedy Quintet, is hardly in the habit of playing Wolf Trap, but it made the most of its opening set. Its tasteful selection of extended jazz and blues pieces was warmly received by the crowd.