I've been looking for over a year now for a personal audio combiner. A wearable device which plugs into my cellphone and my mp3 player at the same time (with different plugs) and combines their output and then the device is also is plugged into my headset. Microphone input from the headset to the device would be directed only to the cellphone, of course.

SkullCandy used to make such a thing - 'used to' - they discontinued it. It even had volume control on the mp3 output.

Why is this device not on the market? Or why can I not find it?

I've tried joiner, combiner, merger, and mixer (which gets all kinds of sound booth and DJ mixers, but none for personal audio cellphone systems).

I had someone knowledgeable in electronics explain to me that this is solvable - just cross the output of the cellphone with the left or right channel output of the mp3 player using a resistor which matches the resistance of the earphone, and also place a same size resistor on the other channel to match the volume. Depending upon the volume loss due to the resistors, I may have to add an amp or pre-amp to each channel, powered by a 9V battery.

I had someone else suggest using a summing amp.

So, I could make this device, but it being SO simple, why is it not on the market? I'd prefer to have it in a nice wrapper ... :-)