Presumably you already checked the volume level is something more than
zero, like mid-way, or higher. When you click on the sound icon in the
taskbar tray area, what it the volume set at? Is the device unmuted?
When you go into Sound properties, which device is selected for Play?
When you run C:\Windows\System32\mmsys.cpl, under the Playback tab,
which device is selected? There may be several listed. Pick the one
labelled Speakers if you are plugging them into the audio minijack
backpanel or frontpanel audio ports.
Since this is a desktop PC, how do you know the external speakers work?
Have you plugged them into an alternate sound source, like your stereo
system (where you have tested the same plug using a pair of headphones
to make sure you hear something from the stereo)? If the external
speakers work with a different sound source, use the headphones that
worked with the stereo to use those on the audio ports on the PC (rear
and front).
Since you have have external speakers, do they have their own volume
control? If so, make sure the volume control on the speakers is about
mid-way, or higher, and make sure a mute control on the speakers isn't
engaged.
When you got the error installing the driver, was this for a fresh
install of Windows XP? Or had you since modified it, like install
anti-malware software, or you restored from a backup image? "Replaced"
doesn't say how you got Windows XP onto the old Dell. "Error" doesn't
say what the error message said.
After an error trying to install the audio driver, did you redownload
the .exe file and try again just in case the first download was
corrupted?
Instead of using Dell's driver, did you uninstall the audio device in
Device Manager and either reboot or do a hardware rescan to use the
embedded drivers that are included in Windows? Even if the embedded
drivers work, check Device Manager again to make sure there are no
yellow exclamation marks.
Into which backpanel port (by color) did you plug the external speakers?
It is possible to use software to change the port assignments on the
backpanel. Did you try the front panel audio jack?