http://www.archive.org/details/OrsonWel ... Recordings
but...http://en.wikipedia.org/wiki/The_War_of ... _ownership
"The estate of scriptwriter Howard Koch owns the rights to the radio broadcast."
Koch would be the radio play's writer. I don't know how it fit into the entire "re-registration" debacle that went on in later years. I see lots of ref's to (c)Howard Koch around, but I can't tell when those pages were written.
Depends on how you want to use it, I suppose. For your own use (non-public playing) you shouldn't lose any sleep over grabbing it from archive.org....