amplifieroperational-amplifier
After replacing op-amp given in the picture
with LM386L the amplifier pretty much stopped working entirely. There is only a faint sound on the output – as if the gain was way below 1.
Given the characteristics and my ignorance I have no dea why this happened.
I think you've got your opamp "upside down" in your schematic though it's hard to tell for sure due to the coarse resolution of the image. But it does look like your feedback divider is connected to the non-inverting input rather than to the inverting input.
If so, you've got something like a schmitt trigger circuit there rather than the difference amplifier you intended.
You have a serious input and output voltage range problem : the input signal must stay within input voltage range (V- to V+ -2V) and the expected output voltage must stay within output voltage range (V- to V+ - 3.5V). Here is what I suggest :
simulate this circuit – Schematic created using CircuitLab
This way :
As Kaz noted, the output drive of LM358 is probably not strong enough for headphones ; you may want to add an output booster stage between the LM358 out and the pot.
If you want to stay with single battery, you will need to create a ground at an intermediate value (I'd suggest around 3V since that op-amp output swing is limited from V- to about 3.5V below V+). For that you can replace the two batteries with a single and create the ground from a resistive divider and an op-amp (the second half of your lm358) as buffer :
Best Answer
Just to make the point clear and actually put an answer to this question. (see all the comments by Dave, Jippie et al.) They are totally different ICs. The 358 is is dual op amp (it has two separate op amps in the same 8 pin package). The 386 is an audio amplifier. They have different pinouts and cannot be interchanged.