The Zaleskis: THE INKLINGS
Glyer: both THE COMPANY THEY KEEP and BANDERSNATCH
Shippey: both his seminal books
Flieger: all three of her major books
Some of what is available is as odd as what is not.
ThusCarpenter's THE INKLINGS is apparently available, yet his BIOGRAPHY is not. Also, some relatively little-known books Xian audiobooks are out:
Bruner & Ware's FINDING GOD IN THE LORD OF THE RINGS
Ware's FINDING GOD IN THE HOBBIT
I'd like to compile a more complete list than this initial rather random dipping, so if you know of others not mentioned above drop me a line and I'll add them to the list.*
P.S. I didn't do a comparable search for CSL and the other Inklings, but I do know McGrath's C. S. LEWIS is available and that I'd recommend it,** while Lindop's THE THIRD INKLING seems not to have an audiobook incarnation.
*and hopefully eventually to listen to them.
**among its other virtues, the audiobook version of McGarth has the surviving bits of Lewis's own voice recordings provided as an audio appendix.