All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is an obligation to keep this trust for its owner and keep it until you give it back to her as it is. Allaah Says (what means): {Verily! Allaah commands that you should render back the trusts to those, to whom they are due.}[Quran 4:58] Moreover, the Prophet said: "Render trust to whoever entrusted you, and do not betray whoever betrayed you.” [Ahmad, Abu Daawood, At-Tirmithi, An-Nasaa’i and Ibn Maajah]
The fact that these tapes include some damaged ones is not a sound reason for you to throw them away or to record something else on them without the permission of their owner, as she might have a different opinion than yours.
Also, you are not permitted to listen to these tapes except with the permission of its owner.
Allaah Knows best.