I have just been using a Y cord that I wired up with a standard 1/8" stereo jack. The input audio is routed to both ears though it will not be true stereo - just the left & right channels mixed together. It's not too bad, certainly better than nothing, does not require an external power source, is small and cheap. I have yet to figure out how to get true stereo into the headset. That would require some electronics knowledge that I don't have and probably some circuitry. When utilizing this kind of setup when you turn on your input device remember to turn up the volume on your communication radios as the output signal is diminished with your input device on. If you don't do this you may miss some radio calls. Of course when your finished with your entertainment if you don't turn down the communication radios your going to get a rather rude awakening.
There are some portable intercoms that will accept inputs and will output stereo to your stereo headsets, and some will function in a manner similar to the Muse.
The easiest and cheapest way to get stereo audio inflight is just to stuff small ear bud type headphones into your ears and put your headset on! Adjust volumes as desired. Works great lasts a long time and their are no interface problems.
What I have found is that when I use the stereo setting on my headsets in aircraft you will only pick up communication outputs from the aircraft in one ear- most aircraft output is mono. If you do this and utilize the Y cord you should be able to get true stereo entertainment in your headset. I don't give any guarantees and your mileage may vary.
I have no problem differentiating communication from the entertainment without having it fade out. Just set the communication volume level slightly higher than the entertainment and you will have no problems.
One more thing, If you are flying an aircraft (most smaller aircraft with ONE audio panel) in which the aircraft audio outputs are paralleled to different positions you will be supplying your entertainment to all the other positions. Your audio input device will have to have enough output to drive the other connected devices. If no other headsets are connected it's not a problem.
Good Luck & Have Fun!