Here are some audio files websites I've stumbled across over the years.

Please note that--
1. "Restricton-free" really means restriction-free except for the usual ones: use cannot be libelous, defamatory, obscene, pornographic, or as part of hate-speech, incitement to violence or lawbreaking; you cannot redistribute the SFX by itself (you have to use it in a production of some sort); you don't get the copyright and cannot claim it
2. I am not a lawyer
3. Use at your own risk
4. If you know of a site not on this list, please feel free to add it
5. There is no #5

Enjoy.

--JK

Freesound.org
Huge and ever-growing collection of SFX, sounders (logos, IDs, etc), music beds in all genres. All contributed by users, all free, all licensed under one of three CC licenses (my experience: about 50% of files are public domain):
+ Attribution/Non-commercial (Use, adapt, remix, whatever; must attribute; no commercial use)
+ Attribution (Use, adapt, remix, whatever; must attribute; commercial use OK)
+ Public Domain

Audiomicro.com
They charge for music, but sound effects are free and royalty- and restriction-free

Awdpro.com/sound_effects.htm
They charge for music, but sound effects are free and royalty- and restriction-free

Freesfx.co.uk
SFX are free, but you must attribute to the website either on-screen, on the label, in a handout, in the website description, etc

Flashkit.com
Click "Sound FX" in the upper-right corner of the page. SFX are user-contributed and come in three licensing flavors:
+ Freeware FX: free, royalty- and restriction-free
+ Linkware FX: free, royalty-free but you must attribute with link to contributor
+ Shareware FX: the cost might be financial, attribution, your first-born--read the description carefully

Mediacollege.com/downloads
In addition to SFX, there are audio test-tones, telephone touch-tones, white noise and production music, all free and royalty- and restriction-free

Partnersinrhyme.com
Site's primary business is selling royalty-free production music, but it also has *huge* collections of SFX and music that are free and royalty- and restriction-free

Soundjay.com
Free, royalty- and restriction-free SFX and production music

"Sound Transit"
Turbulence.org/soundtransit/search
From the website: "SoundTransit is a collaborative, online community dedicated to field recording and phonography." In the public radio community, the genre is often called "found sound." All user-contributed, free and royalty- and restriction-free

Remember these enormous but specialized resources:
The Library of Congress
Loc.gov
Audio (music, SFX, spoken word, live performance), Video & Film, Printed Materials (books & manuscripts, prints, photos, historic newspapers, maps). The section titled "American Memory" is a rich vein of pop-culture, politics, historic and newsworthy events. Not everything is online or downloadable, but more and more becomes available all the time. Almost (but not quite all) is, of course, in the Public Domain

The Internet Archive
Archive.org
Ginormous audio and video sections include every conceivable genre of media, from the beginnings of recording technology to the day before yesterday, 99.9% of it in the Public Domain. The collections are by their nature scattershot, so you may not find exactly what you're looking for, but while searching you may happen upon something even better (and quirkier). You can easily lose track of an afternoon--or a week!--browsing the Ephemeral Films section (including the astonishing Prelinger Archives)

SPECIAL (DIS-)HONORABLE MENTION:
Soundeffectsforfree.com
The SFX are free and royalty- and restriction-free, but be aware that if you sign-up/log-in to download, the site's parent, the pay-site Videoblocks.com, will spam you with come-ons multiple times every single day of the year until the earth crashes into the sun, from a different email address every time--so sending any particular email to your junk-mail file will not prevent future spam messages from Videoblocks