YouTube allows you to watch content on their site. In the case of copyrighted works it is up to YouTube to provide appropriate compensation to the rights holders. YouTube does not grant you the ability to download the audio content of a YouTube video so that you can use the audio where and when you please at a later date. Using a third party utility to circumvent that limitation doesn't legitimise your actions.
See https://www.youtube.com/static?gl=GB&template=terms, in particular:
The following restrictions apply to your use of the Service. You are not allowed to:
- access, reproduce, download, distribute, transmit, broadcast, display, sell, license, alter, modify or otherwise use any part of the Service or any Content except: (a) as specifically permitted by the Service; (b) with prior written permission from YouTube and, if applicable, the respective rights holders; or (c) as permitted by applicable law;
In addition I refer you to section B.8.ii of the Apple Support Communities Use Agreement - Apple Community:
Do not submit software or descriptions of processes that break or otherwise ‘work around’ digital rights management software or hardware. This includes conversations about ‘ripping’ DVDs or working around digital rights management software used on Apple Music or other media sources.
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