In the age of on-demand, digital-streaming music, who has the superpowers necessary to transform a motley collection of AM radio hits from the 1970s into the No. 1 album in the U.S.?

The answer, as summer movie fans know, is Disney’s almighty Marvel Entertainment Group, maker and marketer of billion-dollar-grossing superhero films, including this summer’s “Guardians of the Galaxy.”

Last week, the movie’s soundtrack became the first in history to reach No. 1 on the Billboard Top 200 without including a single original tune. Everything — from the first track, Blue Swede’s “Hooked on a Feeling” to the 12th, Marvin Gaye and Tami Terrell dueting on “Ain’t No Mountain High Enough” — is at least three decades old and easily available as a free download (legal or otherwise) or digital stream.

As impressive as that feat might be, though, hitting No. 1 just isn’t what it used to be. According to Billboard, the “Guardians” soundtrack reached those heady heights by selling a mere 109,000 copies for the weekend ending Aug. 10.

Thirty years ago, in June 1984, the soundtrack to Prince’s “Purple Rain” sold 1.3 million copies on its first day alone. It would go on to sell more than 20 million copies, and even then the album didn’t take over the top slot until early August. (Michael Jackson’s “Thriller,” Bruce Springsteen’s “Born in the USA,” and Huey Lewis and the News’s “Sports” offered up stiff competition.)

Today’s digitized musical universe just can’t compare, saleswise.

According to Variety, during the first half of this year, total album sales in all formats dropped almost 15 percent to 121 million, compared with 142 million during the same period in 2013, continuing a precipitous decline that dates back to the late 2000s. The downturn is probably irreversible: In addition to digital downloads (legal and otherwise), today’s album buyers have multiple streaming options such as Spotify and YouTube. As of Aug. 14, in fact, one of the several available YouTube streams of the complete “Guardians of the Galaxy” soundtrack had run up more than 530,000 views/listens (while a “Guardians”-specific YouTube stream of Redbone’s “Come and Get Your Love” had more than 220,000). It’s possible that the folks enjoying that stream might eventually pay for a copy of the real thing. But more likely than not, a few free listens to Rupert Holmes’ “Escape (The Pina Colada Song)” will suffice for most, before they move on to the next AM radio confection.

Still, topping the charts — no matter the era — with a bunch of songs that were hip to your grandmother is no small trick. Indeed, the demographic genius of Marvel’s superhero films is that they appeal both to the young males who traditionally frequent summer blockbusters and an older generation that grew up with some of those comics (and songs) in real time.

As potential audiences go, that’s a big and lucrative one. All you need is a good mixtape to get their attention.

Adam Minter (shanghaiscrap@gmail.com) contributes to Bloomberg View.

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