Cs55 connects but has no sound on headset

The compatibility setting, in almost 100% of the cases, should be set to "1".  You can find it on the side of the base, it's a little turn wheel that will turn if you stick your thumbnail in it and twist.  I have seen one instance on the Cisco IP phones that requires the compatibility setting to be on 5, so try that as well.
If you have a lifter installed, make sure that the short telephone cable that comes out of the back of the headset's base unit is plugged into the handset port of the telephone (where the handset coily cord is normally plugged into).  You would then plug the handset coily cord into the front of the headset base unit.  Make sure you have that configuration, and try again.
Finally you can try a re-set.  To do this you should lift the headset off of the charging base, make sure the green light on the top of the base is off (the headset is off), and then do the following: 1. Hold down the mute button and the call answer button at the same time. (The mute button is the black little wheel on the top of the headset itself.  If you use the tip of your finger to push that straight in, it will mute the headset) 2.  Keep holding both buttons down until the green light below your thumb on the headset flashes for 2 consecutive series. 3.  Let go.  Wait about 20 seconds then try answering.
If this doesn't work, try a re-subscription: 1. Place the headset back on the charging base. 2. The two microphone volume adjust buttons on the back of the BASE unit should be pressed together.  This will start the red light and the yellow light blinking on the front of the base unit. 3. Hold the mute button down while the headset is still on the charging base.  Hold it down until the red light stops blinking. 4. Disconnect the power to the base so that the red light and yellow light turns off. 5. Plug the power back in after about 5 seconds of it being disconnected and make sure the red light on the front of the charging base is now solid and the yellow light is flashing. 6.  Pick up the headset and try again.

Try adjusting the compatability setting on the side of the base (little black wheel thing you can turn). It may have been dislodged from its correct setting. Aslo check that the cable connections to the telephone are all connected and in the correct sockets.