For those using the Creative Commons license: Yeah, go nuts. Sing over them, chop them up, add crazy effects, whatever. However, you HAVE TO identify what is my work, and what is your work in the credits. Cutting and splicing is fine though, just use the standard credit. For the pro license: you can, of course, edit the track to fit your needs. Cut, splice, speed-up etc. it’s all good. BUT BE WARNED: You can’t distribute these edits as a standalone music track. For example, you sing over it, and then put the song on Spotify/Apply Music, etc. This screws up the copyright system for everyone, as legitimate users will end up getting automated strikes.