I found the circuit on my other computer. What I built is permanently installed in my Skylane, and is in use every time I fly.
When I originally built it, I needed a mixer so that stereo music from an 1/8" stereo cable (Tip, Ring, Sleeve) is combined with three existing monaural sources, one being the HeadPhone bus coming from the aircraft's KMA-24 audio panel, the second being a monaural 4-place Sigtronics SPA-400 intercom, and the third being the Garmin Navigation system "Bbitch-in-the box".
The existing aircraft headphone audio comes the monaural headphone output from an installed King KMA-24 audio panel. I installed the Sigtronics 4-place intercom. Originally, the four headphone jacks permanently installed in the aircraft where monaural, and the previous installer made the cardinal sin of grounding each headphone jack to the airframe, and you could hear alternator whine in each headphone.
As part of the stereo mixer installation, I isolated the four headphone jacks in the aircraft using isolation washers, and rewired them using shielded cable to the output of my home-brew mixer. Look at the attached LTSpice simulation schematic. It shows only one channel. Find the LM386 in the upper right-hand corner.
The LM386 provides a default gain of 20 with a differential input, so effectively sums the signal applied to the non-inverting input to the signals applied to the inverting input. The LM386 is capable of driving a small 8 Ohm speaker, but in this application, even if all four headsets are plugged into the aircraft, I figured that the impedance that the LM386 has to drive is about 37 Ohms (See R6). I determined that a good listening level at the aircraft headphones is about 2Vpp, which the LM386 can easily do since it is powered with 14V. C7 is just an output coupling/DCblocking capacitor. R10-C6 is the required snubber network (see the LM386 data sheet).
The music input to the mixer comes from an 1/8" stereo cable (tip, ring and sleeve). It can be fed from an XM Roady, CD player, IPOD, IPAD, Music Player, or Smart Phone. All of those devices have a stereo jack capable of driving ear buds, which have an impedance of ~35 Ohms, and produce about 0.5Vpp of audio across that load.
Since my XM Roady is powered from the aircraft's electrical system, that effectively connects the audio ouput jack on the Roady to the airframe, which can cause a ground-loop to mix alternator whine into the music. For this reason, for the stereo music input, I used an Argonne AR-170 audio transformer to break that potential ground loop. The music source goes the 40 Ohm winding of the transformer. The schematic only shows one of two channels. There are two of those transformers.
The 8 Ohm winding of the music input transformer goes through an attenuator consisting of R2 against the intrinsic input resistance of the LM386, shown as R3. C4 is there to roll-off the highs a bit... R8 is not real; it is there because all parts of an LTSpice schematic have to have a path to ground...
My Garmin navigation system has a separate audio output which conveys alarms like "obstacle ahead, pull-up, PULL-UP!" in a female voice. I call her the Bbitch-in-the-box. There is a separate audio coupling transformer for that input, which you likely do not need. Ignore all of the stuff on the Left side of the schematic, and ground the Grn wire on the music coupling tranny.
The aircraft VHF radio traffic comes to the mixer on a single wire (See V2, HeadPhone Bus). At the same time I built the mixer and rewired the headphone jacks, I installed a monaural four-station intercom (Sigtronics SPA-400), and its output comes in on another wire (See V4, Intercom). So I effectively have two 0.8Vpp monaural sources to sum into both channels of the stereo headphones. I sum these two signals using a passive resistive summing network (R4, R13, R9, and R1 inside the LM386). There is plenty of level to do this passively because the LM386 has a gain of 20. The summed audio goes to the Inv input of both of the LM386s.
I never drew a real "schematic". I just wired it from the LTSpice sim. The sim was done so I could balance all of the levels, and do some frequency shaping (C1, C3, C4, C7).
This was installed in my Cessna in 2006, and has been in use continuously since. With the Lightspeed Zulu stereo noise-cancelling headphones, the music is very high quality. The transformer-isolated inputs (and proper wiring of the headphone jacks) prevent hearing any alternator whine. The aircraft comms is heard only while there is activity on the radio, the VOX circuit in the intercom mutes any cabin noise when nobody is speaking...
(click to enlarge)