It's a marriage made off-Broadway: Jackie Cain and Roy Kral, the perennial poll- winning jazz vocalists, with Stephen Sondheim, the modern musical's most complex and, possibly, most competent tunesmith.

The flirting, which began 20 years ago when Cain and Kral recorded "Together," from "Gypsy," reaches fruition on the 16 songs and 50 minutes of "The Stephen Sondheim Collection." There's the inescapable "Send In the Clowns" (done in a refreshingly straightforward fashion) and "I Do Like You," a whimsical and witty essay, originally cut from 1962's "A Funny Thing Happened on the Way to the Forum" and revived only when Cain and Kral first presented their Sondheim program at Michael's Pub in New York in 1979.

There are two intimacies involved on this live recording: the empathy that exists between a man and a woman who have shared 30 years of marriage and creative harmony, and the intimacy that exists between an adoring audience and polished performers enhancing intelligent lyrics with intimate and honest renditions.

The triangle of feeling is most effective on Sondheim's love songs, admittedly a little different from the standard fare. He writes of the human condition but without the romantic glaze of older traditions: songs like the restless "Anyone Can Whistle" or the pensive "Johanna" from the disturbing "Sweeney Todd" are unsettling once their message becomes clear. And there are little conversations and exchanges ("Barcelona," "The Little Things You Do Together," "The God-Why-Don't-You-Love-Me Blues") that reflect Sondheim's ear for everyday nuances and anyday absurdities.

There are three modes at play here: sentimental soft-sung solos by either Cain or Kral, energetic unison passages or chases where they confirm their communion. His piano work is subtle and lighthearted, her singing airy and heartfelt, their sense of discovery intact. This collection is as pleasant and refreshing as a clear August sunset. ON RECORD, ON STAGE THE ALBUM JACKIE CAIN & ROY KRAL -- A Stephen Sondheim Collection (Finesse FW38324). THE SHOW CAIN & KRAL, with the Sondheim program and more, Sunday at 4, Hammer Auditorium, Corcoran Gallery of Art, kicking off the Fifth American Songwriters Series.