Hi Erique,
You should be able to see the transmitter's output on a 'scope.
The transmitter frequency-modulates a 44kHz carrier. The Gwahaha receiver tries to amplify the 44khZ but the phototransistor, opamp and your hearing don't go that high. The receiver needs a fast photodiode and an opamp with a much higher bandwidth than just 9kHz like the old 741. A modern opamp like a TL071 has a 100kHz bandwidth and would work fine as a transimpedance amp. The receiver also needs an FM demodulator, not a differentiator.