Hey there!anyone knows of any site where companies regularly post interniship or job positions required?
You could try Mandy.com or craigslist.org (in both the Jobs/Film and Television and the Gigs sections). However, a sound post internship will only appear on either of these sites once in a blue moon. You'd be much better served researching all the studios out there, and approaching them yourself.
Also, i just noticed you're from Guatemala; if you're not a US resident, you might have a very hard time getting a studio to sponsor you for an internship visa. If you post more details about your situation we may be able to help more, but it's a tough industry to crack into.
Best of luck.