By ELVA RAMIREZ and DON SINGLETON

The Radio City Music Hall orchestra's union said yesterday it is filing unfair labor practice charges against the venue's management.

The announcement came on the second day of a labor dispute that left capacity audiences listening to canned music as they watched the Rockettes at performances of the annual "Christmas Spectacular."

David Lennon, president of Local 802 of the American Federation of Musicians, charged James Dolan, chief of music hall owner Cablevision, tried to "extort" him by forcing him to sign a false statement.

"He is demanding that we publish . . . an admission that we lied to the public, press and patrons of Radio City Music Hall, and that is a bald-faced lie," Lennon said. "We are at the NLRB [National Labor Relations Board] and we have asked them to place a complaint against Cablevision."

The musicians walked off the job Wednesday, then tried to play Thursday. But they were blocked from entering the building as Cablevision contended the union was still on strike.