Fixing sound in Ubuntu

Several installs later, I have noticed a recurring issue. You install base OS (in my case Ubuntu, currently on 10.4), and although system sound is OK, I wasn't getting any sound from video clips, youtube, BBC iplayer, etc. Having been through all sort of attempts to fix it, from simple alsamixer adjustment, to installing new drivers, nothing would manage to fix it.

Then some time later I would try to play an .mp3 track stored on the hard disk, and a pop up screen would suggest installing missing plugins, provided by gstreamer. I followed the instructions (all GUI), and sound works in every application and situation. It's only when this happened again, I think for the third time (same sequence of events when installed 9.04 and 9.10 before), when I clicked it's a 'proper' fix, that seems to work for me consistently. Moral of the story - if youhave problems with sound, see if you can play 'standard' music. If not - with any luck you will be guided how to fix it, and you should be in business in a tick!

BTW, I deliberately missed my hardware spec and setup here, as this has occurred on more than one machine.