NEW YORK — Striking Broadway musicians and producers began what were billed as round-the-clock negotiations Monday night.The talks, held at Gracie Mansion at the invitation of Mayor Michael Bloomberg, are aimed at ending the walkout that began Friday. Bloomberg said the strike is causing “a severe economic impact” as well as the “disappointment of theater-goers from around the world.” Eighteen Broadway musicals have been closed by the job action. Major issue in the strike by Local 802 of the American Federation of Musicians centers on orchestra sizes, with producers wanting to cut the minimum size at the largest theaters to 15 from up to 26. The union, which is supported by stagehands and actors who refuse to cross musicians’ picket lines, wants to maintain the higher numbers. With musicians on strike, a weekend without Broadway musicals drove the week’s gross down to levels not seen since the Sept. 11, 2001 session. Twenty-six shows brought in $5,222,462, down $6,183,162 or 54.21% from the previous session. Paid attendance dropped 45.13% to land at 98,773. “Hairspray” led the decliners with a take of $458,671, less than half of what it made the week earlier, followed by “The Lion King” at $426, 415. ‘Rainey’ days Dramas got the big bounce thanks to theatergoers left holding tix to canceled tuners. “Ma Rainey’s Black Bottom,” however, was the notable exception. Despite the strike, the Whoopi Goldberg starrer fell $46,074 to crash-land at an uncomfortable $219,285. But who knows? Since Goldberg portrays a singer who leads a band, maybe some theatergoers thought her show had been canceled, too. At least one local TV station carried footage of Goldberg singing as an example of a shuttered tuner. It could be worse: CNN ran dated B-roll of “The Life” and “Tango Argentino” to illustrate the 2003 strike. Modest upticks In its first full week of regular perfs, “Take Me Out” ($237,248) enjoyed the biggest surge, up $67,682. In its final week, “Frankie and Johnny in the Clair de Lune” ($206,575) bounced nearly as much, up $59,564. Other plays experienced more modest five-figure upticks, including “Say Goodnight Gracie” ($153,686) and “Vincent in Brixton” ($168,210). Up only $6,537, “Def Poetry Jam” ($129,796) could take satisfaction in relinquishing its spot at the bottom of the B.O. Separate union contracts kept open the long-running Kander & Ebb revival of “Cabaret.” Roundabout’s breadwinner removed its discount tix from the TKTS booth over the weekend, causing box office to shoot up $35,878 for a $356,035 finish. Those weren’t astounding dollars, but good enough to land “Cabaret” in the No. 5 slot on Broadway’s Top 10. Lower grosses struck “Flower Drum Song” ($115,400), “Rent” ($63,532), “Urinetown” ($86,343) and “Urban Cowboy” ($39,936) with their truncated performance skeds. More jawdropping were the scores for fractured perfs of “Aida” ($155,677), “Beauty and the Beast” ($131,717), “Chicago” ($178,343), “42nd Street” ($155,143), “Man of La Mancha” ($186,097), “Movin’ Out” ($177,962), “The Phantom of the Opera” ($186,208) and “Thoroughly Modern Millie” ($169,993). In previews, “The Play What I Wrote” chose the wrong week to offer those promotional $1-$5 tix. Four previews brought in $6,893. Strike forecasts There was optimism in the producer offices of tuner “Flower Drum Song.” Producers rescinded their March 9 closing notice, saying the revival would now close March 16 “should the industrial action which has closed 18 musical productions be resolved.” Serino Coyne confirmed that most musicals were pulling their New York Times display ads as well as the ABCs listing for the entire week. The latter ads cost a show between $2,000 and $4,000 each week. Shubert prexy Phil Smith confirmed house managers at six Shubert theaters had been given pinkslips, “to contain costs at this time,” he said.