If you’re an occasional Spotify listener, or perhaps an office bandwidth pig who streams the free music service all day, you’ve probably had a disappointing revelation.

Where are the albums from Metallica? Where are The Beatles and Led Zeppelin? Why is Pink Floyd missing? You can’t even find AC/DC on Spotify — although the Australian hard-rock band did make headlines this week by finally allowing its music to be sold at Apple’s iTunes Store.

These are all rock groups who have decided not to add their music to Spotify’s otherwise staggeringly large music library.

It’s easy to understand why. Despite the anti-piracy ad that Spotify flushes down our throats every 17 seconds or so — the one bragging about how terrific Spotify supposedly is for musicians — it’s difficult to imagine tons of bands making serious bank from a streaming service that doesn’t cost squat to enjoy.

Digital music retailer iTunes employs an entirely different business model — one that can generate real money for musicians — which makes AC/DC’s prior stubbornness head-scratching.

Like, what took so long?

Metallica, The Beatles, Zeppelin and Floyd sell their music on iTunes. Another longtime holdout, Kid Rock, is offering his new album, “Rebel Soul,” on iTunes. (His music also is available on Spotify.)

A few notable iTunes boycotters remain — stay strong, Garth Brooks! — but the idea that iTunes should be shunned is ludicrous. Around 30 percent of music sold at retail in the United States now comes via the iTunes Store.

Get this: Apparently, AC/DC’s hang up had been the idea of splitting an album into individual tracks.

“It’s like an artist who does a painting,” guitarist Angus Young told The New York Times in 2008. “If he thinks it’s a great piece of work, he protects it. It’s the same thing: This is our work.”

If those words were coming from the mouth of Roger Waters describing Pink Floyd’s “The Wall”?

Yes, we would understand.

But this is AC/DC. If a fan wants to resequence the “Back in Black” album so that “Given the Dog a Bone” comes after instead of before “Let Me Put My Love Into You,” I think the group’s “art” will survive.

I love AC/DC. But we also should have the right to cherry-pick, say, “Shake Your Foundations” for $1.29 off the otherwise regrettable “Fly on the Wall” album, which is worth about two bucks in its entirety.

So where does this leave Spotify?

Streaming away, albeit without a handful of very high-profile acts.

And growing fast. Because, frankly, Spotify is amazing.

Its existence seems impossible every time I launch it. Really? You can listen, legally, to almost any song you crave? For absolutely zilch?

It’s depressing how devalued music has become.

Spotify’s micropayment royalty system irks and demoralizes plenty of musicians. (“Yay, my band just made .04 cents!”)

Ironically, it also tends to madden spoiled losers — er, users — who expect everything free. Our sense of entitlement has become ridiculous, thanks to the magical Internet.

It’s human nature to get frustrated when you can’t find a band or song you want to hear, which actually happens more often than you initially suspect on Spotify. The only thing I can imagine being more annoying is if you actually paid for Spotify and couldn’t find AC/DC, Pink Floyd or Led Zeppelin.

Although more than 4 million Spotify users do pay $4.99 to $9.99 monthly for an ad-free premium package, the vast majority do not. They sign up for the free option and tolerate between-song ads.

(Isn’t it freaky how when you lower the volume on your computer, omniscient Spotify instantly pauses the ad and waits for you? It’s like dealing with HAL from “2001: A Space Odyssey.”)

That’s part of the reason why, for now, I’ll use free Spotify, but only as a sidekick to my 30,000-song iPod.

Actually, make that 30,001. “Shake Your Foundations” probably needs to be in my music library.

CARRIE UNDERWOOD PRICES

Thanks to the holiday, you won’t see any major concerts going on sale this weekend. But country fans should be frugal on Black Friday and save up for Nov. 30.

That’s when tickets to the Carrie Underwood show Feb. 17 at Taco Bell Arena go on sale for $43.50, $53.50 and $63.50 at Select-a-Seat.

Those prices are virtually the same as for Underwood’s 2010 Idaho Center concert.

Michael Deeds hosts “The Other Studio” at 9 p.m. Sundays on 94.9 FM “The River” (but not this holiday week); he also appears Thursdays on Channel 6 News.