There are a couple of connection problems with the phone jack.
1) You have your signal ground tied to pin 3, which is normally connected to the tip when a plug is removed, but gets disconnected when a plug is in place. So there is no gnd connection from the input when the plugged in.
2) Your signal input is tied to the ring terminal. This may or may not have a signal on it. If the signal generating app is mono, it will probably be on the tip, not the ring.
Change the ground connection to the sleeve terminal, pin 5, move the signal connection to the tip contact, pin 4.