Check out your local library service's website. All public libraries offer free loans of audio books but the apps they use can vary from one library service to another. (OverDrive and BorrowBox are possibly the most common services though). Then install the relevant app(s) onto a mobile device, such as a smartphone or tablet. Your mother can then listen to them through earphones.
As an example, here's the relevant info for my local library service:
https://www.suffolklibraries.co.uk/elibrary/