I cannot hear it. The two stories are that either Mike threw the bench or the piano itself. I don't think he actually hit Brian but I think he shoved the object at him because he was staring off blankly.
I was actually listening to this very show a few minutes ago. I can hear a few "thuds" right after Lady Lynda, but the recording is quite "noisy" itself. In Brian's "autobiography", it says that Mike threw a chair at him because "He caught me playing a different song than them". I would take that with a very LARGE gain of salt, however.