It is possible that someone has used VCDS to turn off the seat belt alarm - yup, there are some folk with peculiar ideas about certain alerts and there is even a how-to for it which you might want to try in reverse!!
To turn off the seat belt chime: 1) start VAG-COM - now called VCDS
2) click: “select control module” button
3) in Direct Entry box type: 17
4) click: RECODE 07
5) In the Software Coding box should be a number (mine was 0007231). The first digit after the three 0s (7 in the above example) is derived this way:
1 - Brake Pad Wear indicator
2 - Seatbelt Warning
4 - Low Washer Fluid Warning
So if you'd like to be rid of the annoying Seat Belt Warning, subtract 2 from whatever is in that position. In other words by changing the coding from 0007231 to 0005231 you'd get rid of the warning chime! If you change the number to 0001231 you would get rid of the seat belt chime AND the low washer fluid warning.
So enter the number you want and click: DO IT.
6) click: close controller, go back
7) click: go back
However I don't recall anyone wanting to turn the indicator alert off so it may be you do have a duff sounder.
As for replacing a duff sounder read this thread and you will also find a supplier: