Hi Plot,
Hi Zach,
Don't use an old LM324 opamp nor its baby sister LM358. It is the very first low-power opamp ever made. To reduce current, its output transistors run without bias, so it produces terrible crossover distortion! Its high frequency bandwidth is very poor and it certainly doesn't have enough output current to drive headphones.
For years I have been using low-noise, low-distortion and FET-input TL071 single, 072 dual and 074 quad opamps for audio circuits. But they need more supply voltage than a 9V battery.
For battery operation from 3V and more, I use MC33171 single, 172 dual and 174 quad low-power opamps. They are pretty good for line-level audio circuits. Their inputs operate down to ground, just like the old LM324.
If you buy 10 or more pieces of parts from those suppliers, you'll get exactly what you need and the savings over the Radio Shack prices add-up to much more than just for shipping. Newark-In-One promises next-day delivery on orders placed before 8:00PM. You click "Send" on your order and hear the ding-dong at your door just a few hours later.