Newbie. Converting kindle files so I can use voice reader?

Hi there,

Newbie struggling a bit here. Thanks for helping.

I'm dyslexic so I often use Natural Reader to convert text to speech and read ebooks for me.

I recently bought 3 kindle books from Amazon, but I can't get them to convert into anything that will then upload to natural Reader (so rtf, pdf, doc, ePub etc).

Can anyone help me with a workflow to make this work? I tried something with Calibre (not sure I set it up properly) but doesn't work. I also tried an online converter but it failed every time.

I use a Mac with kindle reader. I also have a book Tablet if helpful.

The files I have on my Mac (which seem the right size) are:
.azw
.azw.md

Driving me mad, any input appreciated.

Thanks