Sennheiser 450BT Headphone Warranty Repair Problem

My Sennheiser 450BT headphones no longer produce sound from the right side. I got this product as a gift from a shopping center in another country around 4-5 months ago, and I started using it. Sorry, I threw away the box and everything that came with it. When I reached out to Bircom, they told me to ask about the warranty using the serial number. They recommended sending an email to i***@bircom.com with the serial number to get information and possibly get a replacement or repair. I snapped a picture of the headphone, making sure to capture all the numbers on it, including the serial number. Then, I attached the photo to the email. When I contacted the store from another country that gave me the headphones, they said that if there's no damage, they have to either fix or replace the product. I need you to take the necessary steps to replace or repair my product.