This is a very common problem on Touaregs- it has happened to most of them, including mine. There are two horns, just replace the low one with a new OEM Hella brand low note horn, part #7L0951221. They should be about $20-30, I got mine on eBay.
I respectfully disagree with people suggesting other brands of horns that require cutting off the factory connector and installing crimps. You want the OEM Hella because it will have a watertight connection with the factory connector, and not eventually cause corrosion issues or water intrusion into the harness, especially if you do water fording offroad.