According to the schematic one end of R8 & C16 are connected together to the gate of Q5.
If you break that connection the sidetone will not work as C16 provides the feedback path for oscillation.
Also, you probably need to use standard stereo headphones.
From the manual:
The receiver is designed to drive a stereo output, either earbuds or speakers. If a mono speaker is plugged into the output jack, the receiver will still work, but the sidetone will no longer be audible.
On a nearby receiver, can you hear the signal from the oscillator?
and hear it shift when you key the transmitter?
What are you using for an antenna?
73 - Dave, N4ELM