Headphone amplifier for guitar with stereo MP3 input - mixing
First thing: if directly feeding headphones make sure you don't induce acoustic shock by a signal overload - you don't want to damage someone's hearing so make sure the output voltage is suitably limited in level to prevent this.

For the stereo output of the iPod you need to mix left and right via 1 k resistors forming a potential divider with the centre tap a mono signal. Take that mono signal and feed it via a 10k pot into pin 2 of the 386 chip. Use exactly the same technique as you have done with the volume pot in your circuit; one end grounded and the wiper connected to the original wiper on pin 2. Use a 47 nF capacitor like in the original design and feed that from your 1k potential divider that converts stereo to mono.

That should work after a fashion and be reasonable.


  I also need the stereo input from the iPod to go into the headphones,
  but without any drop in volume for either source


If you really want this level of functionality then use an op-amp mixer circuit like this: -