Midnight is the witching hour--especially when chanteuse Leslie Ball takes the stage in her self-titled open-mike performance cabaret. After priming the audience with her silky vocals, Ball becomes glamorous hostess to a lineup of seven-minute wonders. Anyone can get in on the act: musicians, dancers, poets, comedians, performers who defy characterization. Just talk to Leslie before the Saturday midnight start time for your chance to get in on the act (call the Southern ahead of time, as Balls' schedule is somewhat irregular). And if you prefer watching, at $5 a pop Balls is a bargain, even if the night's entertainment is more Gong Show than Star Search.