Sound - no sound
 

Not exactly a newbie, but feel like a total newbie!
My machine has 3 drives: XP, Mint 13, and Ubuntu 10.04. Sound is fine on the XP drive. On the other two (Mint drive is a brand-new install, btw) test sounds work, but nothing else. Goin' nuts. Ubuntu has gstreamer up one side and down t'other. Zilch on, f'r instance, youtube, or yahoo news sound bites. Nor anything from the CD deck, which plays perfectly under XP.
Using onboard sound chips.

Followup: I went into the Mint system, after reading that I should change to analog stereo in Sound/Preferences, and that worked. I have sound now via Mint, from a Yahoo newsclip. Don't remember which browser, but I guess it doesn't matter.

Still confounded by Ubuntu 10.04. Cannot find anyplace to effect such a change. Test sounds work well, as mentioned. No sound comes from Firefox or Chromium when I go to a Yahoo page, and play a newsclip.