Like the elderly protagonists of ''Where've You Been,'' which recently won her a Grammy award for best country vocal by a woman, the characters who walk through the songs on Kathy Mattea's new album are acutely aware of time. If that's a theme that seems more akin to folk music than to country, it's because no other woman in the country mainstream has so successfully melded folk and country styles. Time Passes By, Mattea's seventh and finest album, is a quiet, reflective acoustic collection. Except for ''Harley,'' a comic tune about a lost baby, these songs are intelligent and poetic ruminations on the glory and frailty of the human condition and of the redemptive power of love. The album ends with yet another take on the much-recorded ''From a Distance,'' Julie Gold's Grammy- winning anthem. Compared to the hugely popular version by Bette Midler, this one is less cluttered, but the inclusion of Scottish pipes makes the song a wee bit precious. On the whole, however, the combination of Mattea's resonant alto, her sensitive reading of first-rate material, and the perfect sparseness of her backing adds up to one of the most eloquent records ever to come out of Nashville.