converted all our shows into mp3 format because mp3 became a standard in the podcasting industry to unlock features such as Dynamic Ads Insertion. 
Mp3 also became a technology requirement for major podcast directories including Spotify. Spotify requested us to convert all our inventory into mp3 to be fully compliant with their guidelines

M4a is not widely supported any more and even though some hosting companies will allow you to upload your audio files in m4a format, they automatically convert each one of your episodes into mp3s anyway before distributing them. 

These changes in our platform are gradually unlocking new possibilities for all the shows in, in the future, for instance, you will be able to add chapter images directly from our dashboard and automatically bake them into your audio files, as well as run different filters to improve the audio quality, and much more!