Check to see if you see your speaker Icon is on the tool bar. If so double click
on it. If not. go to System >Prefs>Sound...

Now that Ubuntu has the Unity interface we really need to know if
users of 11.04 upwards said whether they are using Unity - i.e. with
the left hand side bar. Instructions on how to access settings etc are
very different than with the previous desktop scheme

As for the lack of sound - much can depend on what hardware is being
used, particularly if it's a laptop. So make / model number is a good
start. Also have you tested the headphones / speakers on another audio
device - would be bad luck if both died at the same time but I have
seen instances where that's happened and the poor user has tried to
fix their sound problems in the computer when the fault is external

I have a Dell Dimension 2400. The speaker icon is on
with the sound turned up but no sound.

Seems like the integrated audio on the Dimension 2400 isn't Linux
friendly and you have to resort to a plug-in PCI sound card to get any
sound :-( There are USB sound devices if you don't want to open up the
case to fit a PCI one but some of these are a pain to get working so
overall the PCI option is the better one.

As mentioned - do check out speakers / headphones on another audio
device but chances are they will be fine and it's the sound hardware
in the Dimension 2400 that's not compatible.

I don't have any other device to try out the speakers. "They used to work in
the Dell. We had to move the computer desk and when we set it back up the
speakers did not work anymore.

Now that puts a different light on things. If it was working
before you moved it then look for things like having the speakers
plugged into the correct socket and check the plug / cable to speakers
plus headphones ( BTW - if the headphones plug into the speakers then
try them plugged into the back of the PC where the speakers plug in )

Also look to see if there's already a separate sound card in the
system ( if there is then the sockets for speakers will be lower down
the case, on-board sockets are on the main I/O panel where the monitor
/ mouse and keyboard plug in ). If you do have a separate card then
check that it's still seated correctly in the PCI slot.

I have lost the sound on my Toshiba U205-S5034 laptop after I installed Ubuntu
11.04 too. All the sound settings show the sound working but nothing. I'll let
you know once I figure out where the problem is located.

Strange to say I had the opposite experience with my old iBuddie 4,
now gone to computer heaven. When I used Windows XP the sound was fine
for a couple of years then suddenly the built in sound card was no
longer detected. An external USB sound card fixed that problem, but
Windows never found the internal card again. After 5 years of that I
installed Ubuntu/Kubuntu and suddenly the internal sound card was back
and worked perfectly. That was the case up to the latest Kubuntu
version before I jettisoned the machine a month ago.