The LM386 amplifier schematic is missing important parts and produces an output level into a speaker that will sound the same as the puny level form the original two paralleled opamps:

Granted, but I just grabbed a simple 386 file image from the I'net to give a visual to the point, and did mention input scaling. I should have posted the image currently attached; while adding that bypassing at pin 7 is optional, and AC coupling *is* assumed, though unnecessary in this context.

Nonetheless, I have a quick and dirty kludged audio probe that I use now and then that is not much more than what I illustrated, and it does work well to trace audio signals. The point remains that this can be done with just components that one only need dig-up out of the junk drawer.

 

Now he says his synth will feed an amplifier or an amplified speaker, so I do not know why he overloaded it with an ordinary speaker.

He's coming from the direction of the Arduino community (or so it seems to my eye); which is largely composed of hobbyists who have the barest knowledge of EE, or AE. While I generally support anyone becoming better acquainted with the field of electronics, I also understand that a goodly portion of Arduino enthusiasts lack the formal education requisite to efficiently designing circuits around the 'mini platform'.

Kirchoff, Thevenin, Norton, and maximum power transfer aren't really in their lexicon; although, nothing stands in their way of acquiring such knowledge. Frankly, I'm tickled at the prospect of EE/AE shedding the 'esoteric' and 'too complicated to understand' labels amongst the general population, and am gratified at the number of folks in my local area who have taken the plunge into electronics via the Arduino path.

Not a ding against anyone who is at the novice level in the field; however, when somebody is designing something to interface to an Arduino--or similar--development board, it's a probabilistic indicator that you aren't dealing with an EE or skilled tech, and will have to assume no depth of knowledge in the field.

 

He's coming from the direction of the Arduino community [...] you aren't dealing with an EE or skilled tech, and will have to assume no depth of knowledge in the field.

Yes, I am actually coming from the direction of the Arduino community. I'm still in the middle of my ICT studies and solving (not very realistic) problems on paper is my only experience, so I strongly rely on external advice. Fortunately, I have enough theoretical knowledge to understand why I was wrong, but this is the first time I deal with non-ideal components and I'm not an EE nor a skilled tech.
Thanks everyone for your help!