Comment 45 for bug 253422

It worked for me, too!

I added the line "options snd-hda-intel model=targa-2ch-dig" to the file "/etc/modprobe.d/alsa-base.conf".

Now, headphones and speakers (indeed) share the same volume control.
You get a separate separate window called "Switches" (in Dutch: "Schakelaars") in which you can enable/disable the headphones output. Only now, (in my case) the speakers are muted and cannot be unmuted/enabled, anymore.
I don't care about that, because I don't use sound unless I'm watching a movie or listening to a song, which I only intend to do by using the headphones.

My stuff:
Operating System: Ubuntu Jaunty Jackalope 9.04 AMD64
Computer: Fujitsu-Siemens Amilo Pa1510 (AMD64 processor)
Audio device: ATI Technologies Inc IXP SB4x0 High Definition Audio Controller (Realtek ALC883)