Here I've build an audio CW-filter based on the circuit from NM0S/4SQRP.
This filter works pretty well. I've done some audio analysis using a cheap soundcard from BEHRINGER and the shareware application AUDIOTESTER.
Here are the diagrams. I used the sweep function to perform a transmission measurement: Soundcard-Out --> Amplifier-In and Amplifier-out --> Soundcard-In.
Here the soundcard itself - input/output connected: 
more or less a flat response.
Now the reponse of the first lowpass filter (Between C2 and R4) 
After the second lowpass filter (Between C3 and R6) 
After the third lowpass filter (between C7 and C8) 
And after the final high-pass filter (between C11 and R12) 
The final LM386 amplifier also does some frequency correction, so that the final response of the complete filter is 
Please refer to the original schematic for the part numbers.