the headset jack input (ctia standard) only supports microphone level signals.
you must use an adapter that converts the line level signal to headset level.
warning: sending a line level signal directly into the headset input can damage your device.
note: a regular splitter does not convert the input signal to microphone level. please refer to the technical specification of any adapter before using it to make sure that it properly attenuates the signal to microphone level.