My response was referring to the issue that Engine sound 2 could not be heard. With your current config (Prop #1 assigned to Engine sound 2) Engine sound 2 will only be played by track left or right, whichever is mapped to Prop #1 = SBUS / SUMD Ch. 5.
If you want to use Engine sound 2 for both tracks you must mix them in the transmitter as described previously:
Since Engine sound 2 can be controlled by one prop channel only you would need to mix the channels for the tracks in the transmitter and then use that mixed channel for Engine sound 2.
(You could also use that mixed channel for Engine sound 1 by changing the configuration to single motor.)
Put the file in the Engine sound 2 slot, not Idle sound 2. When you apply the throttle then Engine sound 1 will change to "Volvo aqueeking.wav" and Engine sound 2 to "The roar of the C18 idlee.wav", thus playing both sounds simultaenously with proportional control.
As mentioned above the Engine sound 2 will only be played by track left or right unless you mix them.