This site is very interesting to be able to convert files from one format to another. Above all you can convert MP4 files of works to audio (wav, mp3, etc..) to be able to slow them down when studying them. You will find it on the webhere. And online, so you don't have to download any program.
The next audio converter is very powerful. Convert any audio format to another in a very easy way. It is d’Audio converter Media Human, as I say, one of the most powerful converters, and as always…. free.