This article is from the The Terminator FAQ, by Karsten A. Loepelmann kloepel@connect.ab.ca with numerous contributions by others.
Again, discussion has centred on several possible explanations. In order of
plausibility:
1) The theory from the novelization is that the liquid nitrogen
temporarily damaged its vocals.
2) Another possible reason is that the T-1000 had not heard Sarah speak
to obtain a sufficiently suitable sample; therefore, it could not
mimic her voice.
3) Some have suggested that the T-1000 possesses some malevolence. For
example, it wags its finger in the steel mill after Sarah blows a hole
through its head. Thus, it may delight in torturing Sarah both
physically (spike through the shoulder) and emotionally (helping her
destroy her son).
4) It may be that the T-1000 realized that mimicry was unsuccessful on
John before (when it imitated Janelle). Thus, it may have overestimated
John's ability to distinguish actual human voices from a synthesis.
5) Finally, the best reason (noted in the Special Edition) is that this
odd behaviour is also a result of being damaged by the liquid nitrogen
and being shattered. The SE shows the T-1000 having difficulty
maintaining a consistent form; perhaps it realized it would be
unsuccessful in mimicking Sarah.
 
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