The Fang Dragon (taken from Draconomicon) has an ability called Sound Imitation:
Sound Imitation (Ex): A fang dragon can mimic any voice or sound it has heard, anytime it likes. Listeners must succeed on a Will save (DC equal to that of the dragon’s frightful presence) to detect the ruse.
The DM of our current party has taken this as meaning that this creature can only repeat any sentence it has literally heard while my interpretation is that it can say whatever it wants using the voice it has heard.
The dragon could (with my interpretation) mimic the voice of the king for example to give commands (while being out of sight), granting the listener a will save while with my DM's interpretation it's use is extremely limited.
Which interpretation is correct?