m4b.org
Two readings landing on the same extension without conflict, which is unusual and useful. Context resolves it instantly while the buyer pool doubles.
As an initialism, M4B is the audiobook file format, giving it currency with anyone in spoken-word publishing. On the .org that suits an open library, an accessibility project, or a nonprofit producing audiobooks for people who can’t read print, which is a real sector with real institutional funding.
As an X-for-Y construction it reads as music for blind, meals for babies, mentors for boys, or any number of programme names. The convention is standard in charitable naming.
Three characters, permanently scarce. Enquiries welcome.