On some apps the maximum audio output from the Mac is not enough to drive the FDM-DUO modulation to its selected power limit level. This is especially true when the menu 34 is selected as "MAX" and the output is set entirely by the audio input.
If the desired output cannot be achieved at 100% computer audio output settings, then a program SoundSource can be used to increase the output level of computer audio output beyond 100%, by factors of x2, x3 or x4.
The increase of the audio level can only be applied to a specific application, so this must be chosen first, as shown here
Select the app output audio to go to the USB output FDM-DUO Audio. Then hit to small > right hand icon to show the adjustment menu
Here two adjustments can be made, the Volume Overdrive and the Equalizer. To boost the audio level to the FDM-DUO select x2 up to x4. The volume adjust slider at the top will show the 100% level, and can now be increased up above this by the factor selected.