Can I use any recorded piece of audio in Demix Essentials or Pro?

DeMIX Pro and Essentials are licensed for use for the reproduction of non-copywritten materials or materials in which the user either owns the copyright or is otherwise authorised or legally permitted to use. You are solely responsible for procuring any authorisation and permissions in advance. There are both potential civil and criminal consequences for violating copyright and using and publishing material without proper permission and as per the terms of use in your End User License Agreement (EULA - which you can find here) you are solely responsible and liable for all repercussions and consequences arising from a failure on your part to do so.