Theres alot of ways of doing this. As a I’ve been learning about copyright for the better part of a year now. One of the most simple ways of doing this is to check whether or not the artist is under a large record label or just a record label in general. This is not a fool prof plan however since generally under copyright law when you create something you will automatically have copyright on it. This goes from tracks to 5 seconds to 10 mins in length. Generally large copyright music labels like UMG, SMG, etc aren’t too fond of this. There are some labels that are OK with. I know for a fact that labels like monster cat are OK with this since in their eyes it’s just free promotion. So all in all a good way to do it is to just find out what record label the artist is under, go on their website and find out whether or not their OK with this sorts of stuff. Alternatively you could send them an email asking but this may take a little while. If you do want to use a copyrighted track speeding or slowing down the track is usually a pretty good method. Tough if a label really wanted to they could issue a take down notice or DMCA for using their song/sample in the track. Depending on the amount of content these DMCA claim can be pretty expensive to deal with especially if their a large corporation since they will most likely try to squeeze as much out as possible. So yeah the easiest and safest way to do this is to just find a label thats ok with it or alternatively you could find someone that’s licensed under creative commons. This is what I license all of my song under and it basically means that anyone depending on the type of license Is allowed to share, distribute and transform the material in any shape or form. Plus the artist cannot revoke this license at any time so there is not future copyright hassle. There are several ways of finding these tracks. I know that Soundcloud has a way of sorting trough all of these tracks tough there might be other ways. Hope that helped
BTW I do music. (Contact me)