My understanding of covers (only my understanding mind), UK specific: If you write a song and play it publicly and/or present it in recorded medium it is then legal for someone else to cover it without needing your permission. A few things to note: they must of course present it as a cover, not use any recorded elements from the track (sampling or remixing), mechanicals/royalties must be paid. They have control over their master recording but not your composition for example I think if an ad agency wanted to use the cover version they would need permission from the person who does the cover and the copyright holder of the original, though I'm not too sure on this, either way a lot of people ask if they can use my covers, I say I can only say yes to my version of the composition, not the composition itself. I think this is what
@JohnG is saying above.
Reasons you can stop people doing/releasing a cover of your song are along the lines of defamation etc.
Releasing covers: I clear licenses and pay mechanicals up front (I have to with the digital distributor I use). YouTube did a deal some years back with major record labels, I've not ever been able to find much information about it, but it allows people to earn income from covers, same as you would if you made a covers album. For example, were you to do a cover of an Adele song, YouTube would flag this using an auto-matching database and/or tag/title match and whatever else they use to identify covers. Once the video is flagged you either accept that it is a cover (checking the publisher/copyright holder information is correct), and ads are then placed on the video with 45-55% going to YouTube, a percentage going to the copyright holder (say Universal in Adele's case) and some goes to you, or challenge it if it isn't a cover.
When I reached out to Universal about doing my Adele cover and was interested in using the original vocal, the form did ask for clarifaction on whether it was a cover or an arrangement, I didn't go ahead with it so never got this difference cleared up.
Sometimes I've contacted the composer if I can't find the copyright holder/publisher. For example I did a cover of the Rick and Morty Theme and Ryan Elder was real helpful in sorting me out with that, so there are ways to find out who owns the publishing if the usual databases don't help.
Images and video may present a whole other issue when it comes to copyright. Then educational use, fair use and other things may or may not come into play (according to YouTube's policy only). For example there are YouTube channels that review, react or do 'everything wrong with' or 'how it should have ended', again this is only going on my understanding, they are able to do this under policy laws of YouTube. I think one of those channels had stated that.
A lot of this is case-by-case stuff. Whenever I've reached out to YouTube representatives or spoken to composers/publishers, it's always a little different. So I'd recommend always asking if you're unsure, be sure to find the publisher and clear linceses, make sure mechanicals are paid, do things tastefully and respectfully. My cover of the Walking Dead theme got a nice shout out from Bear McCreary - so I think some people appreciate the efforts of people doing covers of their work. For me, it's a celebration of the music, it's a chance to explore the music and give my own response to it.
So is selling a fan-made sound track legal? If they're covers of the tracks with license/mechanicals cleared then yes.
thought twice before posting though 
