Christopher J. Buccafusco and Paul J. Heald have self-archived "Do Bad Things Happen When Works Enter the Public Domain?: Empirical Tests of Copyright Term Extension" in SSRN.
Here's an excerpt:
The international debate over copyright term extension for existing works turns on the validity of three empirical assertions about what happens to works when they fall into the public domain. Our study of the market for audio books and a related human subjects experiment suggest that all three assertions are suspect. We demonstrate that audio books made from public domain bestsellers (1913-22) are significantly more available than those made from copyrighted bestsellers (1923-32). We also demonstrate that recordings of public domain and copyrighted books are of equal quality.