TCM has an overview. The New York Times has a review. 1000 Misspent Hours says,
Acquiring the film rights to a prestigious story, merely to toss that story away en route to the final cut, is not the only modern-day big-studio behavior of which The Mysterious Island stands as a precocious early example.That site discusses the film being made over a protracted several-years period and then says:
when The Mysterious Island finally limped wearily into theaters late in 1929, it was in the form of a mostly silent film over which sound effects and crowd murmurings had been dubbed with varying degrees of success, interrupted occasionally by painfully awkward scenes of spoken dialogue which play like they were spliced in from some other movie altogether. The final kick in the ass? After all that work and all those travails, The Mysterious Island tanked, and tanked hard.
And yet —incredibly— in spite of all that, The Mysterious Island turned out to be a remarkably effective film.
The following clips aren't labeled so I don't know exactly how much of the film is included here, but this is the most of it I could find online: