what kind of card / computer do you have?

Do you have a little speaker on one of the task bars with a red X next
to it? Sometimes, maybe all the times, the system will boot muted...
and that is assuming you have a card that is supported and the driver
is installed... right click on the speaker and choose "Open Volume
Control"... what do you see? If there are no sliders that make sound
work, try opening Edit>Preferences... you may be able to select some
other tracks to make visible and they may allow you to unmute the
sound.

If that does not help, try to provide the type of card, is it built in
or in a PCI slot... what Kubuntu version are you running... more
details good