Designing a circuit for TRRS Socket
If the goal is to have phones only (not a headset with microphone), it only needs 3-pin 3.5mm TRS socket, so TRRS is not necessary. Sleeve is ground, Tip is left speaker and Ring is right speaker. Output impedance should be fairly low, as typical headphones have 16 to 32 ohms impedance. A speaker would be 8 typically, and there are higher impedance headphones up to 600 ohms. Voltage range is not much, 2 to 3 volts peak-to-peak should be unbearably loud already. There is no modulation, headphones are really just like speakers, the driver elements are directly connected to the connector.