You can monitor your heartbeat , not only the number of it but the precise waveform of it , with conventional earphone or headphone , while you are listening to the music.
Put the sensor on your body and you will find a small heartbeat. By shifting the position little by little, you will find the strongest it. By repetition, the artery trajectory will be drawn on your body.
Pulse wave to pick up from the finger.
Heart beat signal from your finger
You can detect your heart beat from your finger with a microphone with cavity by winded with tape. By simple integration or differentiation, you can get 3 types of the heart beat.