On a customer site, a "normal" (i.e. non-administrative) user cannot access the sound settings (mmsys.cpl) of the computer, whereas an admin can. From the error message the user receives, my impression was that a group policy causes the problem. Do you know which group policy can do so (the customer's admins do not know that...)?
Run gpresult /v from an administrator level command line on the computer that is having the issue. This will show which group policies are being applied and, from there, you'll know which policy's settings to inspect.
ControlPanelDisplay.admx User Prevent changing sounds
A helpful resource are the Group policy object spreadsheets, which document settings by platform. You can find them on the Microsoft download site.