There are 11 total tapes. Three in each of the first three levels and two in the fourth level. Almost all of them are easy to find. You have to listen to them all the way until the end, so if you have all 11 and the achievement hasn't unlocked, try listening to each of them again.
The Beach:
1. Inside the cave, on a table just at the top of the stairs, next to a written letter.
2. On the workbench in the room just past the first tape.
3. Inside the metal box near where you started the level. You'll only get access to this after solving the first puzzle.
The Sewers
1. On a crate on your right as you wind through the entrance part, just down the ramp after the encounter with the professor, where you find the map.
2. On another crate on your right as you wind toward the end of the level.
3. When you approach the hatch at the end of the level, head over to the left into the deeper water and garbage. It's on top of a mattress near some floating paper.
The Lair
1. Down on the ground, underneath a tire in front of the elevator controls at the start of the level.
2. On the table in front of you when you first cross the bridge to the center section (after going down the elevator).
3. On a shelf by some of the electrical components.
The Lab
1. This one starts floating in the air in front of you when you first get off the elevator. Because it's moving around, sometimes you may find it on the ground on the upper level as well.
2. Inside the drawer to the left of the circuit board/control panel.