I have been using this filter in my keyers to clean up the keyer sidetone (converts it to a clean sinewave with roughly 5 msec rise and fall times). I tap off the output of the filter stage to send to Mumble. I mix the audio coming back from Mumble with the sinewave sidetone in the audio amp stage so the speaker / headphones sound pretty much the same as a QSK transceiver. I thought I would post this in case anyone wants to give it a try with their keyers. I really like the convenience for iCW: same keyer / kbd that keys the rig used for iCW; two audio cables from keyer to sound card; and Mumble.
The filter instructions discuss leaving out two resistors (R11/R12 I think, going from memory) and replacing with the 100K volume pot. It is easy to patch in the extra 10K resistor between the pot and the LM386 input, and to patch in the second 10K / pot for the received audio. You then have three audio taper panel mount pots, one for drive level to the sound card and two volume controls for sidetone volume and RX audio volume.