I see questions similar to yours every few days on Y!A now,
so I recently did a whole lot of Googling, searching, reading
and poring over dozens of discussion threads and websites,
only to realize that there are no definitive answers out there.
The only semblance of a part solution which kind of worked
for a few people would require you to either:
1 -- fool around with your "registry" (which I would be totally
deadset against because that could really screw things up);
2 -- download some utilities (which I would be just as much
against, because that kind of stuff is what probably created
your problem in the first place).
Thus, my first piece of advice is to shutdown then reboot. If
that doesn't work, clear your "cache" of "Cookies", "History"
and "Temporary Internet Files", thus making your computer
"fresh" in YouTube's eyes. My infamous "CHTIF" triple-cure
has resolved hundreds of YouTubers' problems. So why not
yours. Please reboot after doing this:
Internet Explorer 6:
→ Tools → Internet Options
→ Delete Cookies → OK
→ Delete Files → OK
→ Clear History → Yes
→ OK
Internet Explorer 7:
→ Tools → Internet Options → Delete...
→ Delete files → Yes
→ Delete cookies → OK
→ Delete history → OK
→ Close (browsing history window)
→ OK
Mozilla Firefox
→ Tools → Clear Private Data
[x] Browsing History
[x] Download History
[x] Saved Form and Search History
[x] Cache
[x] Cookies
→ Clear Private Data Now
Apple Safari 3:
→ Edit → Preferences
→ Security (tab) → Show Cookies
→ Remove All → Done
after that:
→ Safari → Empty Cache → Empty
after that:
→ History → Clear History
If that does not do the trick, here's some other suggestions:
♦ If you've got GWA or any kind of "accelerators", get rid of
them immediately because they make YouTube do strange
things. Also, running applications (like AIM, AOL, ICQ, and
Yahoo "Messenger") can interfere with YouTube.
♦ There could also be conflicts with other video players that
are installed on your computer, like Windows Media Player,
Real Player and QuickTime. Check all of their volumes, too.
♦ The newer versions of Flash are frustrating a whole bunch
of YouTubers. Most updates are typically good for only one
thing: software conflicts. If you are using 1 of the 8 versions
of Flash 10 (yes, there actually are 8), you should consider
the Asker's comments on this other "Best Answer" of mine:
https://answersrip.com/question/index?qid=20081125075929AABZpVS
♦ Who knows why, but sometimes YouTube works better if
you tinker with the URL from "www.youtube.com" to simply
"youtube.com" (or you could even try "ca.youtube.com").
♦ Some people found success by just closing their browser
and restarting it. A couple others rebooted, and one person
wrote to me recently saying that everything started working
magically again the next day with no effort on his part; thus
your problem might just be a temporary glitch.
If you're able to determine the "cause" and its "cure", could
you please click on my avatar and write to me so that I can
include it in my future Yahoo! Answers. One other apparent
cause so far seems to be if you download "Torrents" music.
I wish you luck and hope that there's something in my long
answer does the trick for you.
Here are 2 other possible solutions to videos with no sound.
They might seem old but they both got very positive results:
FIREFOX + VISTA
zarin denatrose . wordpress . com / 2008 / 07 / 31 / fixing - sound - firefox - vista /
http://zarindenatrose.wordpress.com/2008/07/31/fixing-sound-firefox-vista/
WAVEMAPPER
www . my digital life . info / 2007 / 08 / 15 /adobe-flash-no-sound-wavemapper-trick/
http://www.mydigitallife.info/2007/08/15/adobe-flash-no-sound-wavemapper-trick/