This article is from the The Terminator FAQ, by Karsten A. Loepelmann firstname.lastname@example.org with numerous contributions by others.
Again, discussion has centred on several possible explanations. In order of
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.