About legal publishing of a SDL game

So I started a small educational project on C++ with SDL to make some tile-based game so I can exercise some common routines.
However the game turned out to be playable, so I made a level editor and I want to write a website so I can publish it(all for an example educational project, no ads).

Of course it’s freeware and I added the source code files with guiding comments, but since all of this wasn’t planned, at the beginning I downloaded some nice audio and image resources to make the game good in multimedia point of view.
They can be easily replaced by anyone as they are not embedded, but is it OK to add the game to my website, in legal terms? I added the SDL logo and some description about the source of the media files in the main menu.
The graphics are some nice backgrounds found with google images and some random tiles, the sound fx is from some sites for free game audio, and most of the music is from youtube videos of the Portal 2 OST.

So mentioning as much as I can about the origin of these resources if fine, or they are not to be used in third party freeware projects unless explicitly said?

SDL itself is not a problem. The problem is the license of the media files you put together. I think you should check with the owner of the files (including both pictures, sounds and music) that you are authorized to used those files. Otherwise you might get charged for copyright infringement.