Those are mostly free for personal use, not for commercial use. If you want properly copyrighted material, you could register to
The Freesound Project and use the sounds under Creative Commons. Nevertheless, those sounds can't be used for advertizing or commercials other than for the promotion of the work itself, as stated in their copyright statement (
Creative Commons Sampling Plus License). And as always with CC, you need to mention the creator.
Often, for commercial use, you need to buy royalty-free sounds.
As a general rule, it is not permitted to use sounds for commercial use unless it is explicitly allowed under certain rules.
If a
(c) all rights reserved is mentioned, you need to have a written statement to use it (and in some countries it is still not allowed to use certain media, especially songs, even if you have written permission, because you first need to pay dues to collection agencies).
The Creative Commons license is
some rights reserved, which means you can use the sounds for certain purposes (as stated in the type of CC), as long as you mention the creator (this is always mandatory, even for sounds that are deemed public domain).
I'm no legal expert, but some people are just too liberal about use work from others, reasoning that if it is on the internet, it is free game. Reality check, internet is part of the real world and in the RW, creative people are acknowledged for their works.