Freesound Forums

I just used this sound for this video this video and now it's getting monetized by the owner of this song.

Generally, it depends on many things.

If you used some sounds which have the "Creative Commons Attribution Non-Commercial", then you aren't allowed to use those sounds for commercial gain unless you have permission from the author.

It could also be that you used a sound/song from elsewhere which was copyrighted (particularly as you mentioned a 'song', so this very likely might be the case), which would thus have nothing to do with this site.

There is also a very rare chance that some sounds uploaded here were actually from another source in which the sound was under copyright.

If you could give a link to the video, that would help to figure it out.

Defining "Noncommercial" is not so trivial (http://mirrors.creativecommons.org/defining-noncommercial/Defining_Noncommercial_fullreport.pdf).

For monetized youtube videos, it's a good idea to not use CC-ND or CC-NC sounds/tracks. Some people will allow to use their CC-NC sounds in youtube monetized videos, but you probably want to ask first. (Some people/companies sometimes intend "noncommercial" as http://us.blizzard.com/en-us/company/legal/videopolicy.html)