People always recorded Buk - for example, you can see a couple of people put tape recorders on his desk as he reads during Last Straw.
You're right, and I've often said the same thing - he seemed to be recorded at every reading. But at this point we have to assume that most of those tapes are gone or will never be digitized, copied or otherwise see the light of day. I think most people made those tapes for themselves, listened to them once or twice and forgot about them.
I've been looking for "unknown" tapes for years and have only found one. But yeah, there are probably a hundred of them out there still in decent enough shape to salvage, but it's unlikely they'll turn up. Or by the time they do, the cassettes will be 50 year old worthless flakes of once-magnetic dust.
We should be happy for what there is, because there are a decent number of Bukowski recordings available. I've been moving backup files to a new storage unit and I'm starting to realize how many recordings we have of him.
Fans of bands and music in general seem to share all their rare recordings at sites like Dimeadozen, so why not writers?
We don't link to free downloads of things that are commercially available, and most of the Bukowski audio recordings are still available.