Reading this I did a brief search and Kobo looks sort of interesting in offering combined book/audiobooks. A subscription for 1 book a month at $9.99 with no option to buy more credits means I'll be spending a bit more but the extra convenience might be worth it.
EDIT: Audible's media integration and volume levels have been sort of broken on Android for a year now so I've been vaguely meaning to switch and this is just a prompt.
https://www.kobo.com/us/en/audiobooks
EDIT: Audible's media integration and volume levels have been sort of broken on Android for a year now so I've been vaguely meaning to switch and this is just a prompt.