sound problem

I just read the problem below and thought I would see if anyone here can
help me.
Last week my computer failed so I bought a new hard drive and a new power
unit. I found that it was the power unit that had failed so have kept the
old hard drive as it held all my programmes.
Since then I have found that the speakers (they are good quality) give off a
screech or pitch noise at high volumes. I have tried the sound through a
hi-fi unit and still get the screech at high volume.
The power unit is a Hiper, again good quality and has been checked so that
is not the problem.
The computer does not have a separate sound card, it is integrated in the
motherboard, I do not think that I can do anything with the existing sound
system.
Is it possible that the sound element is faulty and if so would fitting a
completly new sound card cure the problem?

Anyone got any ideas?
Thank you.