An unusual pairing of rock and documentary made history four years ago when Melissa Etheridge accepted the first best original song Oscar for a doc (“An Inconvenient Truth”). But this year, as the pic’s filmmakers try to recapture awards magic with “Waiting for Superman,” they’ve found the Academy singing a different tune.

Some major changes were quietly worked into the rules for the song category last year and have cast uncertainty on its future, especially since the days of the signature hit tune appear to be numbered.

Under the new rules, the song category can be dropped if none of the submitted songs meet the scoring standards of the nomination process. In other words, if members of the Academy’s music branch don’t hear enough music they like as they whittle down the possibilities, then there will be one less Oscar to hand out this year, or at least fewer nominees.

In two of the past four years, only three songs have been nominated out of a possible five, which some suggest points to a dearth of quality options.

While “Waiting for Superman” helmer Davis Guggenheim is confident that his submission — “Shine” performed by John Legend — will be among the standouts, he’s aware this could be the first time the Academy forgoes the best song laurel.

The best song award has been a staple of the kudocast, but at the same time has become a lightning rod for controversy. Some critics have complained that in recent years the Academy has nominated songs that most wouldn’t consider to be among the best simply to keep the category afloat.

It’s a concern that Bruce Broughton, chair of the Academy’s music branch, acknowledges. “Some branches have looked at each other — certainly the music branch has on some nominations — and wondered ‘Gee, how did that get nominated?’?” he says. “To have just anything win is pretty horrible.”

More often, however, the Academy has faced scrutiny for shutting out songs widely considered shoo-ins, including tracks by Eddie Vedder from “Into the Wild” and Bruce Springsteen for “The Wrestler.”

While the new rules don’t guarantee that acclaimed songs get any recognition, they are intended to avoid a lackluster slate of nominees.

Academy voters will decide the fate of this year’s contenders in January after participating in either an optional listening session, where all of the songs are played back-to-back over a span of several hours, or by requesting a DVD of all the eligible tracks.

In both instances, each song is presented in its original context within the film, whether it occurs during a crucial dramatic scene or over the closing credits. Each track must clock in at three minutes or less, and voters rate the prospective nominees on a scale of 6-to-10 considering two key factors.

“One is effectiveness, or how the song works in the film,” says Broughton. “The second is how good the song is, in terms of the quality.”

If none of the submissions score an 8.25 rating on the 10-scale, then the best song category will not exist for that year.

Musicians being shut out for a year is troubling to James Austin, producer of “The Envelope Please: Academy Award Winning Songs (1934-1993),” a box-set collection. “That’s the part that bothers me more than anything,” he says. “You have to set a certain scoring threshold or it’s deemed not worthy, which seems to be antithetical to the way the Oscars have been.”

Some industry players have questioned the motivations of the changes, partly because the Acad has come under pressure from broadcasters to tighten the kudocast’s bloated length.

On the flip side, song performances have often been seen as a welcome break from what can sometimes feel like a conveyor belt of accolades to viewers at home. They also served as exposure for relatively obscure musicians, like Three 6 Mafia (“Hustle and Flow”) and Elliott Smith (“Good Will Hunting”).

Others insist the opportunity to celebrate music’s contribution to film is enough reason for the category to exist.

“It’s a part of the process, and that’s why the song category was created in the first place, so we could recognize the filmmaking process,” Tracy McKnight, head of film music at Lionsgate. “You wouldn’t take away the costumes or editing.”

Guggenheim says that when “I Need to Wake Up” won Etheridge the Oscar, it boosted his film’s profile for viewers who wouldn’t otherwise be keen on seeing an environmental doc.

“The song was the carrot that brought people in,” he says. “The emotion and the victory (of Etheridge’s performance) was a huge boost to the movie and to the cause of climate change.”

For now, the unofficial consensus is that the category will be helped this year by a relatively diverse slate of potential nominees ranging from pop and country tracks to more traditional Disney submissions (“Toy Story 3” and “Tangled”), as well as the splashy musical “Burlesque.”

Even if the category is safe, it will always have a sword hanging over its head, Austin says. “It’s a foregone conclusion, it’s not like we’re all voting on something that will come to pass — it has already come to pass,” he says of the prospect of a year without a song Oscar. “One day it’ll happen.”

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