On Ubuntu (every version I tried since about ~2009 or 2008), the sound quality is noticeably different than on Windows XP, regardless of what program I use for playback (YouTube, listening to MP3s, Spotify, etc.)
The sound is clearly worse and less natural on Ubuntu: my laptop sounds somewhat like a tin can (for lack of a better way to describe it), and when the volume gets even moderately high, the laptop case starts to resonate (which is just awful, and never happens on Windows XP, not even at the highest volume).
Please note---this is not a subjective quality difference: the laptop case will resonate on Ubuntu as soon as the volume gets turned up to mid-level. It does not on Windows even at maximum volume.
Some extra info about my setup
The computer is a Dell Inspiron 6000, with "SigmaTel C Major Audio" sound. On Windows I'm using the default drivers from Dell, I didn't modify any settings, and I checked that no extra sound processing is enabled (e.g. 3D or bass boost or whatever that's available on some computers). On Ubuntu 11.10 I'm also using the defaults. There's no noticeable difference in the sound volume of Ubuntu and WinXP.
Why is there a difference in sound quality?
Doesn't the audio data that is sent by programs get "rendered" to speakers as-is? Is there some extra processing, perhaps to compensate for the characteristics of the speakers or laptop case, akin to colour-management?
What can I do to fix this, and make Ubuntu sound as natural as Windows XP on my laptop?