HI there @djrasinbran ,
Well, you're opening the door to a very controversial subject which in fact is not by law and according to YouTube copyright policy, but which became very controversial according to what you can actually see happening on YouTube.
So basically, YouTube Copyright Infringement rules are very clear : you need to have obtained full permission of the original media copyright owner to use other's creation in your videos or you need to use medias produced under full creative common license and for which the copyright owner allows others to re-upload or re-use their material.
There's a kind of "acceptable" and tolerated Copyright Infringement which is describe as "Fair Use" or "Fair Dealing", depending of the country that will status on the infringement, but most of the time, it will end in a case by case investigation and lawyers decision in case of a reported infringement.
But "Fair Use" means to respect a lot of rules and the principle of the "four factors of fair use"
More about "Fair Use" policy here : https://www.youtube.com/yt/about/copyright/fair-use/
Now, coming back to music and DJ Mashups, it's not really going to match those "Fair Use" requirements (even if that seems to work for some people & YouTube accounts ! ), but it's something else and we then come back to the Copyright Owner Infringement rule.
And that's the problem, because it's not about making your Mashup available to download or not, it's not even about trying to monetize them or not, and using written comments like "content not owned" or giving credit to the original artist won't get you out from the risk to be reported and to see your YouTube account suspended for life !!
So in practice, it's a bit of a jungle, and you'll find DJ that made their reputation via YouTube posting mostly Mashups like DJ Earworm in the US and nothing happen to this account, while you find a lot of other's story like this :
Hi, I'm a DJ and I upload my mixes to Youtube with Full credits but they are still blocked! How can I post my mashups? https://support.google.com/youtube/forum/AAAAiuErobUqeUxCtxjihE?hl=en
It's good to know that the copyright owner could be the song creator as well as a label or a major like Sony and so on, and when reporting copyright infringement, YouTube will basically ask them if they want to take the video down or if they want to monetize it, and many times they will accept to monetize it of course, especially if the Mashup rework is having success and reaches a huge audience, because this really happens.
So it's also a matter of business and profit, if Majors and YouTube can benefit from this kind of publications, they will, meaning they will not bother the YouTuber and take the fame and the money ) but if they fill that in the contrary it's not crediting them or targeting enough audience to generate fame or revenues, they will decide ( or not ) to take the video down, and after several times being warned, YouTube could very well shut down the YouTube account definitely.
So to summarize, though the first rule of copyright infringement is clearly saying that you can't use media creation from others without getting full permission or using Creative Commons Licensed medias, what you'll see on YouTube will encourage you to post Mashups videos but in the end you have to know that you are taking the risk to get into copyright infringement and to have your YouTube account suspended for life, so if you rely on it seriously, it's definitely something to take into account, and in the worse case it could even end up with possible lawsuit.
It's a bit the same story about re-sampling, except that the Internet made it so easy to access and because of the mass doing it, it's hard to setup real rules and laws that are just applicable if not on a case by case basis, and that's basically what's happening : everybody does it, some gets amazing benefits out of it, some gets into serious problems.
For interesting cultural video about re-sampling and Mashups, it's worth seeing those 2 videos :
The Amen Drum Break : http://youtu.be/5SaFTm2bcac
RIP: a remix manifesto http://vimeo.com/8040182
Now, this is just what I can tell you on this very controversial subject, many will convince you to do it and that there's no problem with that and it works for them for years, maybe if someone has a rather discouraging but true experience about the other side of things : when it turns to Do Not work, it would be good to hear from those ones too, because in the end it's like the lottery : few winners, many losers...
Now that's just my take and what I grabbed out of it : So I invite others to give you feedback on that subject, especially if they have personal experience, good or wrong with it !
Hope this could help.