Youtube no sound in Firefox browser

Can you give the output of:

uname -a; lsb_release -a; dpkg -l | grep flash; dpkg -l | grep gnash; dpkg -l | grep swf

Thanks

justin@justin-desktop:~$ uname -a; lsb_release -a; dpkg -l | grep flash; dpkg -l | grep gnash;
Linux justin-desktop 2.6.32-24-generic #39-Ubuntu SMP Wed Jul 28 06:07:29 UTC 2010 i686 GNU/Linux
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 10.04.1 LTS
Release: 10.04
Codename: lucid
ii adobe-flashplugin 10.1.82.76-1lucid1 Adobe Flash Player plugin version 10
rc flashplugin-installer 10.1.53.64ubuntu0.10.04.3 Adobe Flash Player plugin installer
ii flashplugin-nonfree-extrasound 0.0.svn2431-3 Adobe Flash Player platform support library
justin@justin-desktop:~$ dpkg -l | grep swf
justin@justin-desktop:~$

Another two questions: Is this problem limited to flash?
And since there are at least two sound systems on linux computers --- one that is portable to other systems and one that accesses the hardware in a more direct way - did you look at all tabs in System/Preferences/Sound?

Problem occurs to all other applications(firefox,seamonkey,opera,movie player,etc) except ubuntu bootup sound ok. System/Preference/Sound only shows hardware as "Internal Audio 1 input/1 output Analog Stereo Duplex", no mention of my add-on soundcard CMI8738. Should I switch back to the onboard sound which is another version of C-Media?

Appreciate your willingness to help. In fact,when I was still running 8.04 Hardy, it was almost problem-free. Only after I upgraded to 10.04 Lucid by fresh install that I get no-sound and wireless networking(TP-Link TL-WN321G usb adapter) problem - the latter just solved.

Somewhat frustrating, otherwise Ubuntu is, in my humble opinion, the best Linux distro and comparable to Windows OS.

I wish to add that my harddisk (20GB Maxtor)is partitioned to dual-boot lucid(16.7GB) with WIN98(3.3GB) and there is another harddisk (4.3GB Quantum)
that I added as storage space which was originally also WIN98 bootable.

Also when I did the fresh install of Lucid, I first deleted the Hardy partition, leaving the original swap space of 773MB unchanged(untouched). So my grub bootloader shows Lucid, WIN98 and WIN98 - the last WIN98 I intend to disable the boot.ini as I only need one WIN98.

One more thing, when I run movie player, I get "PA-stream failed.stop" and then it even disconnects my wireless networking. After closing movie player I could reconnect wireless.

Hope the above info helps. I may hv complicated matters and it's getting annoying as even web browsing takes a longer time to load, on top of the no-sound problem.

sudo dpkg -P flashplugin-installer; sudo apt-get --purge remove flashplugin-nonfree-extrasound adobe-flashplugin; sudo apt-get --purge autoremove; sudo apt-get install flashplugin-nonfree

Should be fine.

If the startup sound isn't working that normally indicates that pulseaudio isn't working. Another candidate would be pulseaudio-esound-compat.

Implementing actionparsnip's instruction didn't seem to work and not understanding what Peter Pall means by his remark - thanks by the way - I decided to switch back to using the onboard soundcard CMI9739 instead of continuing to troubleshoot the add-on soundcard CMI8738, and voila! it works.

Thanks actionparsnip, that solved my question.