Audible needs version 13 or higher to download the app. My iPad mini 2 won’t let me download it because it says it’s up to date with version 12.4.7

Your iPad mini2 can only be updated to iOS 12.4.7.

Apple ended major update support for iPad mini2 in September 2019 - although there have since been a few “point” updates to address specific security issues. Your iPad mini2 cannot be updated to iPadOS 13, or any later major versions of iPadOS, as the internal hardware does not meet the minimum technical requirements for new versions of iOS/iPadOS.

Some App developers have dropped support for iOS versions preceding iOS 13.x for technical reasons (often because APIs or software libraries upon which they rely have been updated in the new version of iPadOS, making their App incompatible with earlier versions). 

Whilst many App titles remain compatible with your iPad mini2, if you want or need to access to Apps requiring the most recent versions of iOS, you’ll need to consider replacement of your iPad with a newer or current model that supports iPadOS13.x.

Sorry, I appreciate that this may not be what you were hoping to learn.

I hope this information provides helpful clarity as to your available options - and the limitations of your current iPad mini2.