>Attr-Noncommercial 3.0

We cant use that one or any derivatives of it. But if the sound effect is worth the effort, perhaps you can contact him and ask if he would release that one file also under the CC-BY 3.0? This apparently is his website: http://www.stephanschutze.com/

I like the windmill sounds. Perhaps make the flapping wings a little louder? It really should be loopable though

Edit: The original didn't make any sense, so I'm going to replace it in full.
All CC 3.0 licenses (besides CC-0) share the attribution (-by) clause; this means you must mention the name or title of the sound, or of the source sound in case you edit or modify it, and the author, in the credits.
You can use unmodified CC-by 3.0 sounds in your work; the sounds stay CC-by 3.0 and you may not relicense them.
You can modify and edit CC-by 3.0 sounds; additionally you may use a different license on the edited work.
You can modify and edit CC-by-sa 3.0 sounds; you may not relicense the sound. This is a strong copyleft license comparable to the GPL.
You cannot modify CC-by-nd 3.0 sounds; you may not relicense the sound. This is a non-free license.
You can modify and edit CC-by-nc 3.0 sounds; you may not use the sound commercially (in the EU, this includes distributing the sound at-cost). This is a non-free license.
You cannot modify CC-by-nc-nd 3.0 sounds; neither can you use the sound commercially (the same caveat as above applies); you also may not relicense the sound. This is a non-free license.

In addition, there's a CC-0 license that tries to approximate Public Domain as far as possible in jurisdictions that don't allow waiving copyright (e.g. Germany).
Sounds licensed under CC-0 or placed in the Public Domain do not place any restrictions on your usage; you can edit it, or use it in whole or in parts. While you aren't required to recognize the author in the credits, personally I believe it to be good form to do so.

Well, first of all, you can read up what it means to license something under cc-by 3.0.

So the only thing you have to do is attribute author of the work. What you then do with it - mix it, built upon it, program a game and use the the work as background art, commercially sell a CD with 1001 sound effects on it - is completely upon you. As with any license, the only thing you have to do is to adhere to it's condition and the CC-Attribution license only imposes this one condition on you (attribute it's author). So to answer you questions:

>If I use CC 3.0 licensed files and mix them together, are they still CC 3.0?
> If I mix a file that has CC 3.0 with one that has Public Domain, how does that work?

Your choice, it's your work now. But of course you still need to attribute the original authors of the mixed files as it was the condition that you may use those files in the first place. With public domain files, you don't even have to adhere to any terms of use.
Normally one doesn't claim an own copyright though when one f.e. just mixes two files into each other, turns the volume down or crops it.

>Is it enough to give links to the sources and write the license type under it?

The source is not important, the author of the work is important. Giving the source can't hurt though as it acts as a proof that you really got it from there and that your claim that the particular file is licensed under the license you wrote is true.