This weekend, I had a couple of the most enjoyable days I’ve ever spent at the Kennedy Center. I watched jazz singer Dianne Reeves guide a high school senior’s transformation from obedient schoolgirl into freewheeling jazz artist in about 10 minutes flat. I was a fly on the wall as the old-school rock artists’ manager who created Metallica sparred verbally with the man who helps create pop hits by putting songs in video games. And I saw futuristic images of lasers erasing a bleeding capillary on a vocal cord as easily as a pencil mark while a doctor explained how he surgically repaired the voices of Steven Tyler, Denyce Graves and Adele.

It was a fabulous weekend — until the singing started.

The “American Voices” festival, which ran from Friday through Sunday, was the brainchild of the opera singer Renée Fleming, but it wasn’t about opera. It was an unprecedented attempt to lift the curtain on the workings of the major genres of American vocal music: pop, country, jazz, Broadway and gospel, as well as classical song. It consisted mainly of master classes and panel discussions that brought some heavy hitters onto the Kennedy Center stages.

Fleming has the superstar clout to involve big stars: Reeves, singer-songwriter Ben Folds, Broadway artist Sutton Foster or the doctor mentioned above, Steven Zeitels. She also has the intelligence and curiosity to make good use of those connections, moderating nearly all of them herself with excellent questions and input. The resulting range and juxtaposition of perspectives bypassed a lot of the tired debates about perceived quality, levels of commercialism and “high” or “low” art. Instead, it offered a look at all singers as serious, working artists and offered audiences a kind of access to high-level people in the music field that even a journalist can seldom dream of.

One of the greatest things about the event, though, was watching artists in action. The master-class sessions I saw — three of the weekend’s six — were uniformly strong: Fleming’s team found some talented young singers from around the country, and Fleming also found master artists who really had something to convey. Operatic bass-baritone Eric Owens won a whole theater full of fans with his amusing and pithy comments (“Sound like Ethel Merman with a little Edward G. Robinson thrown in,” he told mezzo-soprano Deborah Nansteel) but also zeroed right in on what each singer he heard needed. For instance, by having soprano Shannon Love focus on her interpretation, he got her to stop singing flat. Ben Folds was no less helpful; after the young singer Liisi Lafontaine followed his advice to pull back at a big moment in “You Lost Me,” her technically strong rendition suddenly became genuinely moving.

Renée Fleming (Andrew Eccles/Decca)

As for Reeves: “I want you to work with opera singers!” Fleming said to her at the end of the jazz session. I would guess many in the audience concurred.

There were certainly common threads. All of the young artists, in every genre, were told again and again to be individual, to find their own voice, to make their music. This is a laudable goal but not necessarily a helpful piece of advice, since it’s hard to know how to put it into practice. Most young singers I’ve heard are already painfully, palpably anxious to figure out how to be unique in a crowded field, and some, alas, feel that “owning a song” means employing a range of technical embellishments. More useful, I think, were the practical tips to think about the words, about character, about dynamics — to sing, as Owens kept exhorting even the opera singers, as if you’re speaking (something opera singers don’t hear often enough).

But there was also a concert. There had to be a concert, because how else can you cap a festival of voices? It was a big concert, with all the stars, accompanied by the NSO Pops, on the Kennedy Center Concert Hall stage and TV cameras relentlessly panning over the audience and stage. (The festival is being filmed and condensed for PBS.) And its first half, in particular, hit me like a bucket of cold water. Had I seen only this, I would have come away with a wholly erroneous view of this important and engrossing festival as a big crossover mishmash of over­produced vocal pablum.

What made me saddest is that some of the same singers who were telling young artists to be more direct succumbed to a surfeit of mannerisms — none more than Fleming herself, in an overdone version of “Danny Boy.” The usually wonderful ­Owens seemed, in “O tu Palermo” from Verdi’s “I Vespri Siciliani,” to be singing between the notes, although he was much more direct in an a cappella rendition of “Deep River.” This segued straight into a peppy variant of “A Change Is Gonna Come,” sung with technical finesse by Kim Burrell — an unfortunate juxtaposition that signaled the evening’s pervasive problems of taste. Josh Groban crooned reliably in “Smile,” but Sutton Foster was all gloss and bright surface in “Anything Goes,” and Alison Krauss seemed muted in “Ghost in This House.”

The second half was an improvement. I’d have preferred to hear Reeves with a band rather than a pops orchestra (Steven Reineke had trouble getting the orchestra and singers to perform together), but she is formidable. Folds and Sara Bareilles were down-to-earth in “Not the Same” and got the audience to sing along without being too annoying. The surprise highlight, though, turned out to be one of two performers from the previous night’s jazz master class, Kate Davis and Michael Mayo, who stepped in for an indisposed Kurt Elling. Davis, who had seemed mousy in the master class, came out and lit up the stage, accompanying herself on the bass in “I’m Gonna Lock My Heart (and Throw Away the Key).”

Still, after so much meaningful thought and conversation over the weekend about making art with your voice, it was sad that the concert added up to so little — and undermined much of what the artists had said. During one panel, Anthony Freud, general manager of the Lyric Opera of Chicago, observed that “we seem to be losing ability as a society to distinguish between the excellent and the successful.” The concert illustrated exactly what he meant. I found it all the sadder because the festival as a whole was such a fine reflection on Fleming, who here used her celebrity creatively to do something important and useful for the field, and I wish more of Saturday’s audience had gotten to see that side of the event. “The quest to perfect how we bring beauty into the world is never-ending,” Owens said during one panel; the weekend allowed a fortunate audience to watch the quest in progress.