This is kind of a funny thread; "Tiger Rag" is such a great record, but there's more apparent interest in a stray sound in the run-off area than in the music itself. Nevertheless, I guess I'll just post one query: why does there seem be an assumption that the voice heard belongs to Bix? Though we'd all love for that to be true, we simply don't know. It could be anybody.
But for all you audio sleuths out there, here's another lesser-known moment: the run-in area of "Santa Claus Blues" by the Clarence Williams Trio (Okeh mx. 73721-A) where we can hear a faint countdown ("one - two - three...") before Louis' solo pickup. It sounds like someone whispering.