With performances at the Ontological Hysteric Theater and an exhibition at the New Museum of Contemporary Art to its credit, 31 Down radio theater seems well qualified to be experimental. And true to experimental fashion, its production of That's Not How Mahler Died takes great risks as it attempts to explore the themes of death, voyeurism, and guilt. The payoff for these risks, however, is a definitive failure. Which is not to say that production values are lacking. Nor is the crew in any way untalented