Actually I think I've just got it. If I change the speed of one track to save it, then I guess the default speed changes on every other track I select to play. So I need to save them all at the speed I want and then play them all at 100% (so they're playing at 100% of the speed I saved them with). Sorry but just playing around with it for the first time.
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I have heard of people using youtube-dl to extract m4a files from YouTube videos and using those. That is against the TOS of YouTube so we can't endorse that.