Connect RIN and LIN to the line out terminals of a CD player, cassette deck or whatever you use as the music source. Connect ROUT and LOUT to the line in terminals of an amplifier. If you have a mono amplifier, just connect either ROUT or LOUT. Note however that the music source must be stereo.

If you play a mono CD track or tape, RIN and LIN are the same, so RIN - LIN = 0. You can test this by temporarily removing R12. You should hear no sound. If you disconnect LIN or RIN, you should hear the music.

Having R12 still removed, play a stereo song. You should now hear the music, but no vocals. Otherwise, the vocals are recorded in stereo as well; try another song (preferably from another CD or tape).

If you hear that the vocals are gone, you can put R12 back in place. If you hear the vocals again, the low pass filter may not have been properly dimensioned. Experiment a little with R11 and C1. If you can't get it right, you may need to add a second filter (just between R11/C1 and the + pin of U2.A), making the attenuation 40dB per decade.