I too doubted that they were deaf, so I went and did a little research into the China Disabled People’s Performing Art Troupe. As it turns out, not all of them are completely deaf. Some of the other performers in the the troupe, especially the musicians (pianists, flautists, singers, etc.) are blind or physically handicapped.

The English version of the official site has a lot more information:
http://www.mydream.org.cn/about.htm