"Roughly one is five American households listened to an audiobook within the last year—23 million households. (Audio Publishers 2001 Consumer Survey)"[2]

"'The best patrons are the best book-buyers. They’re avid readers who use audiobooks to keep up when their eyes are busy,' says Mary Beth Roche, president of the Audio Publishers Association. ('Commuter Consumer,' The Washington Post, April 24, 2005)"[2]

Households and adults aren't exactly comparable, but it's a start. That last quote supports my personal, anecdotal findings that most audiobook listeners also read books.

1: https://www.edisonresearch.com/audiobook-revenue-and-the-num...

2: https://web.archive.org/web/20101119164743/http://audiopub.o...