This is an ancient thread, but it is where I looked for help, and after figuring it out, thought it is where I'd be most likely to look when I've forgotten how, so here it is:
To correct the problem of no sound in limited user accounts when the primary administrative user account has functioning sound in Windows XP, first check the sound card settings from the administrative user account and write down every setting from every tab. This will vary from soundcard to soundcard and I can't tell you what the settings for your soundcard should be but if sound is working for the administrative account, those settings in place on the administrative account are adequate, and duplicating them on the limited user account with no sound should get you half way there. These settings may be available from an icon in your System Tray at bottom right of the screen, or may be available from Control Panel (Start/Control Panel/Sounds and Audio Devices/Audio Tab). The following is tedious but it should work if you reboot after each step.
From the administrative user account, change the limited account to an administrative account. To do this, Go to Control Panel/User Accounts, select the limited account, change it to an administrative account, and close User Accounts.
Log into the formerly limited, now administrative, user account that has no sound, and open Control Panel and open System. Select the Hardware Tab, the select Device Manager, then from Device Manager select Sound, video and game controllers in the menu or click the plus sign to open that menu to show your audio card. Right click your audio card, select Properties, and on the General Tab, look to the bottom to make sure that Device Usage menu has selected "Use this device (enable)." IF not, select Use this device (enable). Click OK and close everything.
Follow the same steps outlined above for the administrative user account to view the settings for the sound card, and make sure they are all the same as appears in the notes you took from the original administrative user setup. If they are not, change them to match and click Apply and then click OK and close everything out.
Log on as the formerly limited, now administrative user that had no sound, and see if it now has sound. YouTube is a good test. If it has sound this worked, if not, you need to ask someone else because this is all I know, but when you are finished, whether it is fixed or not, unless you want that user to have administrative priviledges (probably not or you wouldn't need the user account), switch users to the original administrative user and change the user that had no sound back to a limited user.
Log in as the original administrative user and double check to make sure the the user that had no sound is now showing as a limited user. Switch users to log on as the once again limited user and test for sound. If you have sound, you are done, and if you don't, well I tried!