Allan,
Due to the lack of an oscilloscope, I managed to put an IR receiver near the cassette AC and I received the same command I sent.
Furthermore, I managed to control a TV and a sound system by my very same circuit. Hence the wonder whether my circuit is not allowing the IR LED to control the cassette AC or it's a software/ command related issue.
I always thought the raw command received by the receiver should work fine if you transmit it at the right frequency. Pardon my ignorance on the matter, but why did you suggest attaching an oscilloscope across the LED?