>>393753932
They're scattered all throughout zones in pretty easy to miss areas. Sometimes they'll be in crates and boxes, othertimes just sitting on the ground next to a dead body. When you pick them up you can either play them in your cassette player in your pullout map or play them back at base at the mission console. I don't think there's anyway to play all your previously collected tapes while out and about though. Only the cassettes you come across in the moment.