NOTES FROM THE UNDERGROUND That Downhear Lounge just never knows when to quit, thank god. Continuing a tradition of bold and groundbreaking entertainment, tonight's lounge offers a multimedia performance by the aural pranksters known as the FOUNDATION FOR PUBLIC BROADCASTING. Also on the plate: film projectors, live and prerecorded music, Matt Goff on the didgeridoo, and a special appearance by JC Hopkins and the Van Lucious Memorial Orchestra. Kris the doorman might even turn up too. Top that. 9 p.m., Cafe du Nord, 2170 Market, S.F. $3. (415) 861-5016. (Neva Chonin)