Electronic – How to switch this audio jack, using its own mechanical switches, without creating feedback

The TRRS connectors are wired like this, in order:

• Tip: Left output
• Ring: Right output
• Ring: Ground/return
• Sleeve: Microphone input

That way the headphones will work plugged into a normal TRS jack, too.

The microphone input is meant to connect to an electret mic with a circuit like this:

The bias voltage, resistor, and DC-blocking capacitor are all inside the phone. So just connecting a signal source like a computer's sound card to the microphone input (with ground to the ground ring) should work.

If your sound source's DC output resistance is too low (normally a good thing), the phone could be interpreting it as a continuous button press (Answer or Play/Pause), because the headset button shorts the mic line's bias resistor to ground. Does it do anything to indicate that it thinks a button is being pressed? If this is the case, just put an electrolytic capacitor in series with your source, like 10 µF to 100 µF between your source and the microphone input. This will block the DC current from the bias resistor while allowing the AC signal to pass. Larger capacitance will have better low-frequency response.

For instance, if the input impedance of the phone is 2.2 kΩ, 10 µF will have a -3 dB cutoff frequency of 7 Hz, which is plenty.

Sounds like you should consider placing some good quality capacitors in series with the audio signal lines from the jack that you added and to where you connected the inputs to the guts of the old cassette player's amplifier input. I would start with a value of 10uF with a voltage rating of 16V or more. It would be best to utilize non-polarized capacitors but if you cannot find those then consider one of two options:

a) Use two 22uF capacitors in series wires + to + and then one - lead to the jack and the other - lead to the cassette input.

b) Try to see how a polarized cap would work. Put the + lead toward the cassette player amplifier and the - to the jack.

These capacitor ideas block the DC bias that is apparently present at the connection point where you attached the signal wires into the cassette amplifier board. Some of the earlier devices that you connected to the cassette jack may very well have had capacitor coupled outputs. On the other hand your newest device may not have an output configured like this.