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I can't comment on the iPod, but there's generally no need for any external resistors, as they are normally fitted internally anyway.
Most modern personal electronics, IPODs, CD/MP3 players, FM stereo radios, use IC audio amps that have an effective output impedance of << 0.1Ω. If you short the left channel to the right channel, the two amplifiers end up fighting each other, causing severe distortion, and drawing excessive current from the batteries, causing the batteries to go dead very quickly. Putting the bridging resistors externally is necessary!!!
Battery-powered personal electronics devices that run on 3V batteries are designed to drive ear-buds or headphones with an impedance of ~30 Ohms as the normal load. Since the peak-to-peak amplitude of the audio is so constrained by the battery voltage, the last thing they want to do is to put a series resistor between the output of the amplifier and the earphone...
Your sketch does not show a volume control. Instead it shows a variable resistor to ground that shorts the input signals. A volume control does not short the input signal.
Even with the pot set to zero ohms, the amplifiers in the IPod are protected because they see a 22 Ohm load. The LM386 likes a low source impedance. I'm guessing that for a comfortable listening level, the pot will be set near max resistance, and that the user will be using the volume control on the IPod, anyway...
btw-I've been using this exact circuit in my airplane for ~10years... works just fine.
I'd try the direct connection to the headphone first. I'd add the 386 only if you need more loudness.