Electronic – “Stopping” sound after a certain frequency

A fair share of the spectral content of most music is below 500-1000 Hz, and with a fairly weak first-order filter falloff of 6 dB/octave, your waveform will be relatively unaffected. A much higher-order filter would be required, preferably at a much lower cutoff (440 Hz is concert A, bass hits should be way below that), with envelope detection (a diode and cap).

As you don't care about the actual fidelity of the audio, a passive filter should suffice, though the order required may significantly reduce your amplitude and reduce the effective resolution.

Addendum:

Just to reiterate other's concerns, the main problem of just filtering audio like this is that if you have some very loud hit (e.g. cymbal crash), it will still go through your LPF (low-pass filter) and give you spikes that you may interpret wrong. Another problem would be inability to cope with much dynamic range (music getting softer/louder); though you may be able to add some variable threshold.

As you've correctly stated, this approach will work best with Techno, but that still might not be very good.

Adding to @GeorgeHerold's answer:

1. You need to include frequencies down to below 30Hz. Most of the energy in a typical heartbeat is between 30 and 40Hz. Given the cutoff for the highpass that @GeorgHerold computed, you are losing a good 15dB at the lower end.
2. Try using a directional microphone instead of an omnidirectional. Directional microphones have two openings, one in front and one in back by the solder pads. I find I get much better results by installing a directional mic such that the front (where you normally speak into the mic) is inside the tube, with the port on the back of the microphone out in open air.

Actually, I don't use a stethoscope in my experiments. A simple plastic shell with a hole in it for the microphone works very well. What I've used for this is a plastic cap like you would use to close one end of a plastic water line. I had some laying around that are about 2" in diameter, and they work well.

I find that a directional electret microphone installed in a platic cap as described works well enough that I can capture heart sounds using nothing more than a PC or smartphone with no additional electronics - I can then do filtering, recording, or other processing digitally.
The PC soundcard has a 20dB boost on the mic input, but the smartphones don't. Since the cards use 16Bit sampling, you have plenty of headroom to digitally amplify the signal even without using the boost. If you resample from 44100 down to 400 or so, then you could probably gain the equivalent of a few bits