"..We aren't lawyers so this isn't legal advice, but here's our summary: for the "zero" license you can do pretty much what you want with the sound. You could even sell the sound, ... but you can't claim you are the author! For "attribution" you should always mention the original creators of the sounds when you use them. "Noncommercial" works like attribution, but you can't earn any money with the piece of work you create! As with all licenses the original creator can give you permission to use the sound outside of the original license...
..an additional license called Sampling+: Our interpretation of this license: you can do pretty much what you could do with the attribution noncommercial license, but additionally you can't make commercial advertisement with the sound. You can't make a track with Sampling+ samples to sell a car, for example. Sampling+ is being removed by creative commons because it's a difficult license to interpret.."
You are also encourage to upload your own sounds for others to use..
All the details are at https://www.freesound.org