DRM free does not mean copyright free. You can quote small portions of a copyrighted book for review purposes but trying to convert the entire book to an audio file is not something that any business that wants to continue operating will do. Copyright is not removable though it can be transferred, it is the rights the creator has in their creation.
At one time, you could use a calibre plugin to convert an ebook to mp3 using the
TTS to MP3 but I have never used that plugin nor do I know the current status of that plugin.