Which everybody kept mispronouncing as the “Oh-Need-Ers.” Jimmy was too clever for his own good.
Heavy metal band In This Moment played in season 2 episode 1 of the TV show Murder in the First. The lead singer even gets one dialog line (she calls the bad guy a limp dick freak when he jumps up on stage and points a gun at her).
I started to post about them. In a way, they’re fictional—that isn’t their full-time job, it’s more like an acting gig. But in a way they’re real because their skill is legit, they’ve had concerts and recorded. Therefore I think you can argue either, but they’re either both fictional or both real, and neither of those fits the OP. In fact, the two groups played some of the same concerts. Wikipedia says:
When the trio subsequently toured as Spinal Tap during 2001, they would occasionally perform in the guise of the Folksmen as an ostensible “opening act”; not all of the audiences appreciated (or even understood) this in-joke, with one appearance in New York City reportedly being booed by a restless audience.
and
In a 2009 interview, Guest reflected further on the phenomenon:
One time, we had The Folksmen open for Spinal Tap because we always wanted to do a culmination of our entirely different personas. So there we were in caps playing folk music, opening for Spinal Tap, and the audience looked completely bewildered, like “What the fuck is going on here?” It was great. My son was at the show, and asked, “Mom, when are the old guys getting off and loud guys coming on?” That may have been a moment of weirdness for some people, but so what?[4]
Scrubs fans may already be aware that Ted’s a cappella group were an actual a cappella group called The Blanks.
Sadly, Sam Lloyd (Ted himself) died last year of cancer.
Barbershop quartets count? The Mayor and school board in The Music Man were the Buffalo Bills.
Would the Tubes’ performance in Xanadu count? They were never named within the story. I think Fee Waybill was credited as “80s Rock Singer”.
In the 1994 movie China Moon, a band is playing the blues song Tell Me What I Want To Hear. The band is listed in the credits as “Blues Band”. It is actually Anson Funderburgh and the Rockets featuring Sam Myers, and the band is the best part of the movie IMHO.
Nitpick: in the 1962 film, the Buffalo Bills (Bill Spangenberg, Scotty Ward, Al Shea, and Vern Reed) played the four feuding members of the school board. Mayor George Shinn was played by Paul Ford, who wasn’t a member of that group (and I don’t know that he was a singer, either).
The Buffalo Bills were also the quartet in the original Broadway cast of the show.
Thanks, Kenobi.
You are welcome! (It’s my favorite musical, so I obsess about it more than I should. 
)
In Robert Rodriguez and Quentin Tarantino’s vampire movie From Dusk Till Dawn, the house band of vampires in the ‘Titty Twister’ bar is the real life band “Tito and Tarantula.” They also wrote and played the song that Salma Hayek dances to in the movie.
Texan punk rockers Bowling For Soup performed the theme song for Phineas and Ferb, and appear as themselves in one episode.
Jaret Reddick, the lead singer of BFS, also voices (and sings as) Danny, the lead singer of Love Händel, a fictional band on the show.
The thread may have drifted toward examples of bands playing themselves? I was going to post all the appearances at The Bronze in “Buffy the Vampire Slayer” (like The Breeders, Aimee Mann, and others), but they weren’t labeled as fictional bands (in fact, one of them had signage with their real band name), and generally they played their own songs.
The real-life English rock bad Slade played the fictional band Flame in the film Slade in Flame. This was no cameo – the members of Slade were the top-billed actors in the film, which was about the creation, rise, and fall of an English rock band.
In Everything Is Illuminated, Gogol Bordello has a cameo as the Ukrainian folk band playing at the train station. (Their frontman, Eugene Hütz, is one of the lead actors.)
Franz Ferdinand was supposedly invited to play the band Weird Sisters in Harry Potter and the Goblet of Fire, but it ended up being a mishmash of band members from Radiohead, Pulp, and a couple of other bands.
The band also was never named in the movie because of an ongoing dispute over the name with Canadian folk band Wyrd Sisters.
Sha Na Na was the band that played at the prom in Grease, but were called Johnny Casino & The Gamblers in the movie.
And in the recent live remake, DNCE (of “Cake by the Ocean” fame) played the band.
The Donnas appear in Drive Me Crazy as The Electrocutes.
That’s actually really clever.
I suppose it’d be OT to mention the Buffalo Bills in the Music Man…?