The phone was connected to the analog inputs of the MPC, which is independent from any OS
(I assume a cable from headset out to line in)
The phone may not detect any output device if it checks for headphones connected.
A line input has a much higher impedance, which appears as „nothing connected“ to the phone and it deactivates the output amplifier.
(this is not necessarily related to activity of the output meter)
If you have a headphone socket „duplicator“ (to use 2 headphones on one phone), you could plugin earbuds and use the other connection for the MPC.
(it‘s not a proper solution, but could verify if impedance is the source of the problem)