recently I stumbled on this on Reddit: during the GDC 2015, the Sonniss music library made a huge (10 GB) audio bundle for anyone to use with any project (including commercial, yay !).
Since I haven't seen a topic mentioning it yet, I thought I would share it with you. This bundle contains a great variety of ambient music and sound effects of all sorts.
My understanding is that you can pretty much do anything you want with the audio, including modifications and using it for commercial projects. The only requirement is giving credit to the authors of music you use.
OTOH one thing you can't do is re-selling it as your own (seems pretty obvious but... ).
Currently the preferred way to get this seems to be by torrent : click to download with your torrent client of choice.
Excerpt of the license agreement :
Have fun browsing itGrant of License
Up on downloading this sound effects collection, the Licensor grants the Licensee, a worldwide, non-exclusive, royalty-free license to use all or any of the sound effects in accordance with the terms and conditions laid out in this Agreement.
a) Licensee may use the licensed sound effects on an unlimited number of projects for the entirety of their life time.
b) Licensee may use and modify the licensed sound effects for personal and commercial projects without attribution to the original creator.
c) Licensee may freely distribute the licensed sound effects and make an unlimited amount of copies.
d) Licensee may publicly perform a reproduction of the sound effects over any form of medium.
a) Licensee may not modify any of the sound effects with intent to claim authorship of the original recording.
b) Licensee may not sell any of the sound effects as they come. (Although the sound effects may be sold as incorporated into licensee project).
Let us know if you spot something I didn't see in the license that would badly prevent using it in any circumstance (e.g. for commercial projects...).