Installed 12.04 on a new HP Pavilion desktop p7-1235 (AMD A8-5500 APU). Volume is very low from external speakers and headphones plugged into the front jack. I set the master volume and speaker/headphone volumes to max, but volume is very low. Used alsamixer to set everything to max, unmuted, same result.
There's a chance that your sound card might not be driving enough electricity to up the volume. If it goes lower when you reduce it, this may be a good indicator of that.
The best suggestion would be to get a set of externally powered headphones/speakers.
Source: I had an HP DV-9000 with the same issue about 2 years ago.