Ok, on Freesound.org, next to the sound, there is a small collection of icons with things like people, dollar signs etc in them. Hover your mouse over these are the licence details. The details are different for each file. A quick google search of the name of the licence will bring up it’s terms.
They all seem to be under various Creative Commons licences, so the worst restrictions possible will probably be:
- Can’t sell them
- Attribute the persons name
- No modifications
As you say it’s a free game, you won’t have issues with the Non-Commercial clause, so you may just have to provide back-links and credits.
Likewise, freemusicarchive.org also has little icons, again with the same creative commons specific to each individual file.
So go and look at them, and see what the license says you must/can/cannot do.
On a side note, I nearly got caught in this trap about a year ago, and since then I’ve decided that if I plan to distribute anything, I will make all of it. As a result, I’ve had to learn how to draw with Gimp and make my own textures, I’ve had to learn how to create my own sounds with Audacity, and even write my own music with LMMS. (All the software is free!). The result is that it takes an extra few weeks to make a game, but it means there are no license issues.
On the rare occasions that I really can’t make something myself, I either triple check the license before I use it, or contact the creator. If neither of those is possible, I find a different, similar resource which I can find those things. Typically, this is for GLSL shaders these days, as I need to learn that side of game making still.
Everything is copyright to the original creators, so the blend may be yours, but the resources may not be. It all gets confusing, which is why I just steer clear of the whole mess.
If you are desperate, and there aren’t to many sounds, PM me links to them, and I’ll see if I can create you some similar sounds. I’ll release them to you under the WTFPL license. I don’t feel like writing must at the moment though.