This circuit won't differentiate between one sound and another, that's much more difficult to do.
It sounds like you're doing it the hard way. Why not connect an LED and resistor across the injector coil, or if you don't want to make physical connection, use a coil to detect the injector coil's field.
From the circuit descritpion:
U1b operates as a low-noise Sallen and Key, Butterworth low-pass-filter with a cutoff frequency of about 103Hz. R7 and R8 provide a gain of about 1.6 and allow the use of equal values for C3 and C4 but still producing a sharp Butterworth response. The rolloff rate is 12dB/octave. C3 and C4 can be reduced to 4.7nF to increase the cutoff frequency to 1KHz to hear respiratory or mechanical (automobile engine) sounds.
I have a sepehoscope from harbor freight I can just place it on the running injector and hear it "snap" on problem!
I dont like wearing it around moving parts and 100K volts!!
Some of these injectors are hard to get to! Infact some engine dis-assembly is required to get to them, and thats just to check them for mechanical movement. Doing this with a prob using sound or light short-cuts this whole process, for the shade tree DIY'er!
I did some experment today with a earphone,I hooked the ear phone to my meter and a red LED , when I tapped the earphone the LED blinked dimmly and got a reading of about 29.0 MV, AC. Tap was more like a slap!
Thanks for your thoughts!