The Master Mystery photoplay is in the public domain and has been reprinted several times. But now LibriVox Audiobooks has recorded an audiobook version complete and unabridged. The recording comes in at a whopping 7 hours 52 minutes and can be listened to in it's entirety for free. The reader is Roger Merlin.
You can also download a full version in various formats along with cover art and CD insert at the LibriVox website.