If you're running beta v1.7.0.12 or higher, you can use the "Wait For Spoken Response" action which is designed to do exactly this.
Otherwise you'll have to use you can use dictation, though that's much more cumbersome, and less likely to actually recognize what you're saying(especially with short words).
E.G.
Start Loop While : [DictationComplete] Equals False
Start Dictation Mode (Clearing Dictation Buffer)
Start Loop While : [{DICTATION}] Equals ''
End Loop
Stop Dictation Mode
Begin Text Compare : [{DICTATION}] Contains 'Yes'
Write [Purple] 'Yes' to log
Set Boolean [DictationComplete] to True
Else If Text Compare : [{DICTATION}] Contains 'No'
Write [Purple] 'No' to log
Set Boolean [DictationComplete] to True
Else
Say, 'Please say "Yes" or "No"'
End Condition
End Loop
EDIT: Exergist makes a good point, in that you can also use a separate "[yes;no]"(or similar) command to set a variable that's monitored by your command, though I'd also recommend either using the beta or waiting for the full release that included the "Wait For Spoken Response" action.