If you are playing a game with 5.1 surround then all speakers should be working.
If you are playing a stereo cd or mp3 then you will only get stereo.......unless you have selected to up-mix stereo (and 4.1) sources to 5.1 sound.
You can do this in your Audigy Control Panel.
I'm using an X-Fi card so my Control Panel settings are different from yours.
If you cant find the setting you need I'm sure an Audigy user on the Forum will soon be along to point you to the right setting.