Do you think it's an invasion of privacy to listen to this cassette tape?

Back in 1994 I was 11 and my brother was 16. Him and his group of friends were ‘troublemakers’ in the eyes of my parents and they were concerned. They set up a tape recorder that would record incoming and outgoing phone calls to monitor his behavior, and long story short- I’m now in possession of the tape from that recorder.

I always thought the spy-phone thing was a little extreme in this case….there weren’t any violent crimes being committed, they were pretty much just sneaking out at night and smoking pot. I only listened to the first 5 minutes of the tape so far, I just think it’s interesting to hear what I sounded like when I was 11. Do you think it’s unethical to listen to this tape?