I had a similar problem when I installed Ubuntu, my PC speakers would
work, but when I plugged in external speakers into the headphone jack
I got no sound at all. What I did was install the GNOME ALSA Mixer
which can be found under Add/Remove Applications, if you can't find it
just search for alsa and it should turn up. Once installed you'll find
it under Sound & Video, open it and you'll see all these sliders
controlling the volume of different sounds. I ended up turning the volume
up to max for everything then playing with the check boxes until it
worked. Hope this helps.