Lengthy efforts to reach a compromise on New Orleans’ controversial sound ordinance suffered another reversal Thursday, with the latest draft revisions rejected by the City Council on a 3-3 vote after nearly three hours of debate and a slew of last-minute amendments.

The vote leaves a major question mark over hopes for revising an ordinance that dates from 1956, and which all sides agree needs an overhaul.

Mayor Mitch Landrieu’s administration still wants the law changed, as do various neighborhood groups. But three of the seven council members will relinquish their seats before the next council meeting, and that changing of the guard could alter the politics surrounding the issue in ways that are hard to predict.

The sound ordinance emerged in the past few months as one of the most sharply debated in city government, as council members, the administration and community groups tried to hash out a compromise that would rein in the worst volume offenders without damaging the city’s music culture or running afoul of free speech rights.

The ordinance up for a vote Thursday would have established new caps on decibel levels along Bourbon Street, set specific limits for bass-heavy sounds and shifted responsibility for carrying out the law from the Police Department to the Health Department, among other changes.

Yet ironically, Thursday’s vote turned on one component of the noise debate that had not come up much, at least not in discussions at the council: a curfew, already in place, that prohibits anyone from playing a musical instrument in a public street after 8 p.m.

The ordinance would have eliminated that curfew. Landrieu’s administration wanted the curfew jettisoned because of concerns that it might be unconstitutional and would attract lawsuits. Street musicians wanted it gone as well.

But French Quarter neighborhood groups, already opposed to the latest version of the ordinance because they want even tighter volume limits, seized on the issue this week. In an email blast, the Vieux Carre Property Owners, Residents and Associates organization accused the mayor and council of ramming the change through without debate, pointing out that discussions over the past few weeks have explicitly focused on Bourbon Street noise levels, not citywide policies.

The group found an ally in council President Jackie Clarkson, who seemed alarmed that a set of revisions purportedly aimed at reducing noise might actually weaken enforcement in some respect. “All of a sudden, this masterful document that was going to be the cure-all is a total surprise to half of us who have been involved, and a total 180 from where we were going,” she said.

Clarkson, along with Councilwoman Susan Guidry, offered an amendment to maintain the curfew, but it could not attract enough votes to pass, and Clarkson refused to vote for the ordinance without it.

Councilwomen LaToya Cantrell and Cynthia Hedge-Morrell joined Clarkson in voting against the proposal. Both argued the council should take more time to iron out differences — and confusion — before committing any changes to law.

Councilwoman Stacy Head, who had expressed support for the latest revisions this week, was absent, which left Kristin Gisleson Palmer, James Gray and Guidry without the fourth vote they needed to carry the motion.

Gray spoke up the most vehemently against the curfew, saying it might only heighten friction between police officers and young black men.

Clarkson’s amendment would not only have left the curfew in place but also would have broadened what types of sound it applied to — an effort to meet concerns that the existing ordinance might violate the U.S. Constitution’s guarantee of equal protection under the law by singling out only those with musical instruments.

“I think it would be a terrible mistake for us to pass a law that says if you’re making a sound with a device in New Orleans after a certain hour, you’re breaking the law,” Gray said.

Members of the public — nearly two dozen spoke over the course of the meeting — weighed in from every angle.

Danielle D. Leger, a spokeswoman for the Louisiana Restaurant Association, said, “Our restaurant patrons should be afforded the right to enjoy their meals without the disruption of street musicians who may set up outside of a restaurant with no time limits.”

Street performer Roselyn Leonard sang her way to the microphone and told council members that existing laws are already being misapplied. “I have been arrested singing on Royal Street at 2 in the afternoon on a Sunday.” she said. “I went to jail. It took me seven hours and three judges and three lawyers to get me out.”

While Thursday’s vote hinged on the issue of the curfew, most debate up to now has centered on how high to set decibel limits and how to measure them.

The revised ordinance, for instance, would have set a cap of 92 decibels for street-facing properties on most of Bourbon Street after 10 p.m., or 102 decibels for more bass-heavy sounds. That would replace a fuzzier provision in the existing law that allows for 10 decibels above the “ambient sound level” of the street.

Other changes would have made the limits more easily enforceable. Health Department employees, rather than police officers, would have to take a sound reading for only 20 seconds instead of 10 minutes. And the readings would be taken at the property — usually a nightclub — producing the loud sounds instead of at the site of a complaint — usually a nearby residence — thereby making it easier to identify the right perpetrator.

In a bid to corral enough votes at the 11th hour, Palmer introduced a set of amendments that would have delayed implementation until January, as well as clarifying that residents would still have the right to sue noise violators and police would still have jurisdiction to enforce the rules if need be.

It was not enough.

Whether any of this can be salvaged will now be up to the new council members scheduled to be sworn in May 5.