In the guide I will tell you how to find the secret cassette and the way to the cassette player in the secret room
Where Can I Find the Secret Cassette?
In the Combine Nexus chapter ((map pht_a15_nexus_a), at the very end, after we fought off a wave of combines and they threw gas bombs into the room, you go back from where you came from and a door opens there, to the left of the door, you will see a table and on it you will see a business suitcase with a cassette that you open and just take the casset with the button E
After you with cassette are bent down together, don’t go forward, but turn back and see a doorway with plywood that blocking the entrance, you can destroy the plywood with all weapon and then go inside the room
After you have destroyed the plywood, what blockaded door , you go inside and just insert the cassette into the player, sit and just enjoy the secret.
Thanks to Alikersantti for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.