Nope. I did the same thing myself with my Spectrum. I guess I was 14. I didn't have a microphone so I tried it with my dad's stereo headphones. I got the idea to try it after looking at the Z80 machine code of cassette saving and loading routines. Maybe it was also after ETX came out, first game I remember with speech samples. (Terrible game). I forget the details now but it's all about the timing between turning the mic/speaker on or off.