There are only six cassettes, all of which are at your work place in the following locations.

Cassette One: Ground floor, women's bathroom.
Cassette Two: Ground floor, warehouse. All the way to the right of the ground floor.
Cassette Three: First Floor, first door on the right.
Cassette Four: Second floor, second door on the right,
Cassette Five: Third floor, second door on the right.
Cassette Six: Win the Anarchy Man mini-game.