Have you looked in Device Manager to see if there are any flags against
the audio entry there?
If she recently ran Windows Update and applied one of the fixes, and if
the machine has a Realtek chip, then loss of sound can be a consequence
of that.
Have a look at:
http://support.microsoft.com/default.aspx/kb/935448/
But more likely is that XP as she got it does not have the drivers for
sound? Check the emachines website and see if they ever supplied that
model with XP on it and if there are sound drivers to download.