The non-free flash plugin can be a bit of a headache, since even if you install it and flash videos play nicely, you may not get any sound. On a laptop, you might not notice it, since the default output seems to be the laptop speakers. However, you may have problems piping the sounds through any attached USB speakers. Same goes for desktops, where the default output may be the sounds ports on your motherboard.
The fix is simple (assuming you have pulseaudio installed)
create an /etc/asound.conf file or a ~/.asoundrc file, with the following in it:
Rebooting should take care of things