Generally, onus is on both the sides to consult any expert to opine on the veracity of the evidence. As you said that plaintiff has not taken any opinion of an expert to present the veracity of the evidence, you can definitely go ahead and consult an expert to present your side on the veracity of the tape. When both the sides don't consult any expert on the matter, the court has to step in and do the needful.
So, you can definitely go ahead, consult an expert and present those facts in your next defence.
Refer this case and other cases listed in the same
best of luck,