In this case both mp4 and m4a (which are the same thing) use AAC. So no transcod...

deadf00d on Feb 4, 2022 | parent | context | favorite | on: I hacked SONOS and YouTube the same day

In this case both mp4 and m4a (which are the same thing) use AAC. So no transcoding, but mp3 is not. So in the case of mp4 > mp3 it need a complete re-encoding of the audio.