ABOUT HTTPS://WWW.YOUTUBE.COM/WATCH?V=RTJLZTH-FLC

As you can see there are lots of things listed without any movie resolution but with text "audio only", and they are the ones you're looking for. If you understand you might be permitted to achieve this (You aren't breaking any rules like copyright by undertaking this), you can use youtube-dl to down load the supply audio information only, devoid