I see questions just like yours every couple days on Yahoo about vids with NO SOUND,
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 am really against,
because that could screw up your computer);
2 -- download some utility (which I'm also against, because
that kind of stuff is what maybe caused your problem in the
first place).
As such, I advise you to first clear your "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. Be sure to shutdown/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 3:
→ Tools → Clear Private Data
[x] Browsing History
[x] Download History
[x] Saved Form and Search History
[x] Cache
[x] Cookies
→ Clear Private Data Now
Mozilla Firefox 3.5:
→ Tools → Clear Recent History
Time range to clear: → Everything
▼ Details
[x] Browsing & Download History
[x] Saved Form and Search History
[x] Cookies
[x] Cache
→ Clear Now
To clear your "cache" within IE 8, Safari, or Google Chrome,
please check out this other "Best Answer" of mine:
https://answersrip.com/question/index?qid=20081125075929AABZpVS
♦ If you got Firefox and insist on using Flash's latest player,
make sure you include it as an add-on to your web browser.
♦ 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 my avatar and let me know -- so that I can
include it in my future Yahoo! Answers.
I wish you the best of luck and hope that there's something
in my long answer that will do the trick for you.
Here are 2 of the other possible solutions I described at the
very top. They might look old but they got positive feedback:
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/