This allows us to double it up and have separate Left and Right channels, as shown here and below.
Step 1: Some sample videos
Step 2: What's needed
LM431 Programmable Zener
You will also need a means to plug into the earphone socket of your music player and a male-"A" USB plug.
You can perhaps see an earlier prototype in the rear of the image below. There I've modified a perf board as a USB plug. It works, but definitely is not recommended unless you know what is involved and you have an inexpensive USB port to test on.
Step 3: How does it work?
We are using this precise voltage to bias the LED just below its Vf, in fact you will see a faint glow even with no signal.
The input pin is monitored by the 'reference' pin and any changes in the signal shows up as fluctuations in our LED. The 15K resistor controls the sensitivity and can be as low as 3.3K if you like your music loud, or if your phones are less efficient. The LED brightness is controlled by the 120-ohm resistor.