Best way to see what Q1 (the mute gate FET)
actually does is to disable it and listen.

Before proceeding, be aware that audio
from headphones may end up being mighty
Tack-solder a temporary jumper across
R9(4.7MEG) on the bottom of the board.
This will effectively "bypass" audio
directly into the headphone amplifer, disabling
the FET from its mute function. Won't hurt
the FET ;-)
You should notice no difference
while receiving; the FET is normally about
0.1Kohms while the key is up. 100 ohms is as
good as a short-circuit compared with the
22K of R10.
But now when you hit the key, be
prepared for a monster sidetone. The FET
becomes a very high resistance once the key
is down, but audio just shoots thru the
short-circuit you've added instead. See how
far back you have to turn the RF GAIN pot
to cut the sidetone level.
With the short-circuit removed, a
little audio gets past the FET through R9,
providing a reasonably comfortable sidetone
R8 and C24 are only in the circuit
to provide some delay on key-up before the
FET turns "on" and the receiver's audio
comes back up. They have nothing to do with
audio level, only timing.
However R8 is required to drage Q1's
gate up to the same voltage as its source.
Vgs=0 volts is required for the FET to be "on".