Frequency Change with Tape Speed I recorded my own voice in an old tape recorder. When I put the device in fast forward mode my voice turned a little squeaky. I wonder if the fundamental frequency of the voice had changed? Is that possible?

Yes, it is possible. If it is playing faster, that is outputting the analog value that used to be at time $t$ at time $t/a$ with $a \gt 1$, for some constant value $a$, all the frequencies have been multiplied by $a$.