When Paul Simon and Art Garfunkel reunited for the first time in 20 years at the 2002 Grammy Awards to sing “The Sound of Silence,” their fans issued a collective gasp of surprise (and appreciation). One of the seminal folk-rock groups of the 1960s, the pair had long ago engraved itself on America’s collective consciousness. In six albums over five short years, Simon & Garfunkel released hit after hit: “Homeward Bound,” “America,” “Mrs. Robinson,” “The Sound of Silence,” “The Boxer,” and the list goes on. On “Old Friends: Live on Stage,” an album that collects performances from their 2003 reunion tour, it is a pleasure to hear many of those songs sound as good as ever, and time has only added to their haunting beauty.

As the record’s liner notes explain, after years of tension in their relationship, and in the wake of Sept. 11, the two decided to come back together for a reunion tour, saying they had put aside their differences. The concerts, unfortunately, were hit-and-miss. The two played all their hits to enthusiastic audiences, but seemed somewhat awkward and lifeless on stage, just going through the motions. Fortunately, their performances work much better on album; rather than bored, the pair sounds reflective and elegiac. Though their voices can’t escape inevitable aging, years of experience and personal conflict give their songs more weight.

The opening song, “Old Friends/Bookends,” with which Simon & Garfunkel began each concert, sets the wistful tone of the album. The group has wisely not changed the arrangement of the song: Simon’s soft acoustic guitar backs the gentle melody. But the entrance of the two aged, raspy voices takes the song to a different level — almost to the transcendence of Johnny Cash’s recent “American Recordings.”

In their twenties, on the 1968 album “Bookends,” they sang: “Can you imagine us, years from today,/ Sharing a park bench quietly?/ How terribly strange to be 70.” Today, with the singers close to that age, the line lends the song a solemn nostalgia.

The quieter tunes are the undeniable highlights on this album. Aside from a few added instrumental lines and relaxed guitar and piano solos, the songs’ arrangements remain nearly the same as always. Fewer electric guitars and less percussion place the focus on the two singers, where it belongs. Their renowned harmonies have become imperfect approximations of what they once were, though the songs don’t necessarily suffer as a result. Obvious exceptions are “Mrs. Robinson” and “Cecilia,” on which their voices lack the energy they had in the 1960s.

The Simon & Garfunkel classics “America,” “Slip Slidin’ Away,” and “The Only Living Boy in New York” are heartbreakingly beautiful. On “American Tune,” a Paul Simon solo composition, Garfunkel naturally weds his voice to that of Simon, improving upon the original. Only occasionally do the slow tunes feel inert and plodding; “Bridge Over Troubled Water” is particularly disappointing and uncompelling.

And what of “The Sound of Silence,” the classic that symbolized and defined a generation? The two have quieted the opening of the song nearly to a whisper. The moving meditation on identity and despair is stripped down to just the two aged voices: It is a gorgeous closer to the first of the set’s two CDs.

Annoyingly, Simon & Garfunkel are content to let many of the faster songs stand on their nostalgic merits alone, rather than attempting to breathe new life into them. Even with the help of the Everly Brothers’ appearance on “Bye Bye Love,” their sound merely recalls a past era, failing to transcend the trappings of old-timey corniness.

Where “The Concert in Central Park” (their classic 1981 reunion concert album) is joyous and invigorating, “Old Friends” is piercingly quiet and mostly melancholic. But the change seems appropriate, and the new live record stands as a worthwhile companion to the first.

“Friends” shows that Simon & Garfunkel’s contemplative music has stood the test of time. As most of the album’s 23 tracks prove, their songs have achieved a timelessness that will last the years.