Some soundcards occasionally exhibit an issue whereby the speech synthesis module causes a lockup of the speech recognition module. When this occurs, the Radio Call Light will remain yellow, even when the aircraft has finished speaking. Subsequently the speech recognition will appear not to work.
Selecting this option in the Speech Tab of the Setup Window will mean that a press of the PTT (Press-to-talk) button will clear the lockup. For most systems, there is no need to have this selected.