Changing Speed to a WAV so it is still understandable


I have a WAV voice recording and when I change the speed down to minus 20 it sounds awful and incomprehensible. How can I change the speed of the voice recording so I can still understand what is being said but is slower so I can transcribe it. In Quicktime player, at 50% reduced rate, it is still too fast for me to translate and type so its need to be slower but still understandable.

Any help would be wonderful as I have 30 one hour interviews to transcribe and it’s going to be painful !

Bye Caroline