UPDATE: I know this tutorial sucks, but it’s hard to make it better as I’ve not had these problems lately nor do I have much more patience to work on it. So, it’s here if it helps, if not I’m sorry. I’ll do the best I can to get a good one eventually.
As evidenced by the numerous threads on this at the official forums, these are all quite common and unfortunate issues. I don’t wanna steal traffic from them 
, so please visit them and ask the good folks if one of these solutions doesn’t help, or just because I told you to. These solutions come from there anyway, I just want to conglomerate them into an easy-to-digest format. I’m only directing you, this won’t solve everything. At the most all I can give you is links.
Let’s get down and dirty, let’s Flash each other:
First off if you don’t have Flash installed already, get the official Adobe nonfree version by typing:
sudo apt-get install flashplugin-nonfree
Now most of the time it should work flawlessly, though a lot of people have problems.
The symptoms:
- No sound in Flash applets
The solutions:
Try this in a terminal..
mv ~/.asoundrc .asoundrc.old
mv ~/.asoundrc.asoundconf ~/.asoundrc.asoundconf.old
From what I can tell this recreates the configuration files (for ALSA?) anew. I know for sure that updating on top of an already installed system (eg Gutsy -> Hardy) can leave gunk behind. I think that’s why YouTube used to freeze so much as well. So, I’d advise you to try this, especially if you’ve performed an upgrade instead of a clean install.
If this doesn’t work:
Install alsa-oss and set Firefox to use it
This may cause latency in sound by a half-second or so.
Sources:
http://ubuntuforums.org/showthread.php?t=590989
http://ubuntuforums.org/showthread.php?t=255422
The symptoms:
- Flash videos freeze
- Videos perpetually load (specifically YouTube)
- Other Flash objects freeze/don’t show up
The Solutions: There isn’t one that I can find. I haven’t had any problems with this as of late but :/
The symptoms:
- One is unable to play a Flash video with sound when another app is playing sound at the same time.
The solution: Figures, this is a PulseAudio problem. Just install libflashsupport:
sudo apt-get install libflashsupport
This corrects Flash 9’s glitch of not outputting sound through PulseAudio correctly. Be advised though that at least for me it had a tendency to crash Firefox a lot. After so much crashing I’d rather not have multiple sounds at once frankly, I uninstalled it and Firefox hasn’t crashed in days (save for a couple mp3 problems)
Sources: I found it somewhere on Ubuntu Forums at first, but I don’t remember where, nor can I find it. But thank you whoever you are! 
Other solutions
Having a different issue or none of these worked for you? Try these solutions.
- Use an alternative Flash plugin (Gnash, swfdec) — Gnash didn’t work too well for me; I’ve never tried swfdec.
- Random crashes: http://ubuntuforums.org/showthread.php?t=778498
- Search the Ubuntu Forums! It’s worth it. I’ve fixed 99.9% of my problems by nosing around in there.
And thus it ends. Linux is a beauty and a curse when it comes to diversity. I hope I could help somebody by gathering all these posts together. More will be added over time, but there’s simply too many solutions to list here. Please feel free to suggest additions to this post. I wanna help as many people as I can
Goodbuy.
