Is it allowed to use sounds from people unknown to public?
Sounds that are available online on various sites…
Are you asking if you can steal someones song because they are not known? 
I’m just joking here. But you ‘could’ read the question that way.
You need to license the work regardless of how popular or known someone is.
As far as “Sounds that are available online on various sites…”
All of these sites have license statements and if they do not assume you cannot use them.
Okey. I am not talking about songs only here 
But I got your point (site licensing terms)