In order to determine what is wrong with your headphones, utilize this troubleshooting guide.
The ATH-AD500 headphones were first available in the summer of 2004. They were the first consumer priced headphones from Audio Technica. The headphones began Audio Technica's foray into consumer priced, open-air, headphones. The AD500s were followed by the AD700 and AD900. Although this product is now discontinued, these guides will help you repair the ATH-AD500 headphones.
When trying to identify the ATH-AD500 model, look for a silver grill and black body. Also, above the grill on both sides of the device, the make and model are listed as: Air ATH- AD500.