Please elaborate what (and how) you are exactly doing.
Do you use UI controls from Windows or are you actually writing your own software to control the volume of each PCM2704?
The PCM2704's, if attached through a hub to a PC, will not be aware of each other but will be controlled by the Windows sound subsystem and the Windows USB class audio driver. If the volume changes for all of them (not just for the one that was addressed, something is wrong on the Windows side (if you wrote your own program, please make sure that you have a unique (and correct) file handle to each PCM2704C.