Because the Dell Optiplex 380 is too new to Ubuntu, so the drivers of sounds is not really well.
After I update the OS from 9.10 to 10.04, there is no sounds!
But at last, I found the way to resolve this.
1) Open the terminal, insert the code below
wget -c ftp://ftp.kernel.org/pub/linux/kernel/people/tiwai/misc/hda-verb/hda-verb-0.3.tar.gz
2) Compile and install the hda-verb
tar vxzf hda-verb-0.3.tar.gz
cd hda-verb-0.3/
make
3) Add two lines in /etc/rc.local, so that they’ll be automatically applied when booting
/path/to/hda-verb-0.3/hda-verb /dev/snd/hwC0D0 0x1a 0x701 0
/path/to/hda-verb-0.3/hda-verb /dev/snd/hwC0D0 0x1b 0x701 0
Remeber to replace “/path/to/hda-verb” with the actual full path of your “hda-verb”, as to me, I locate it in /home/david_lee/, so my code is:
/home/david_lee/hda-verb-0.3/hda-verb /dev/snd/hwC0D0 0x1a 0x701 0
/home/david_lee/hda-verb-0.3/hda-verb /dev/snd/hwC0D0 0x1b 0x701 0
Now you can restart the computer, and enjoy the music! Good luck~
More details : https://bugs.launchpad.net/ubuntu/+source/alsa-driver/+bug/582199