Electronic – 4N35 opto-isolator for MIDI input does not work

Disclaimer: This is in response to the posted circuit, not an answer to the question of control with a PWM. I just didn't have room in the comment.

Any special reason you need the optoisolator? As your circuit currently appears, I don't think that your D5 connection to pin 5 of your optoisolator phototransistor will supply the kind of current you want. In addition, synchronization between the two PWMs will likely cause you pain (What if the D5 PWM is always off when the D3 PWM is on and vice versa? No sound! You need to average your signal, and feed it into a buffer. You'll also likely want some hefty capacitance on your V+ line to the micro and buffer so that your supply voltage stays smooth.. However, the connection to ground and the connection to D5 totally eliminate the purpose of the optoisolator, which is to allow the speaker circuit to be at a different voltage than the input. This might be useful if you had the speaker at a remote location from the Arduino, and want to run a low-current differential signal on a twisted pair out to the speaker, which would be powered from a different power supply. As is, you might as well just connect pin 6 to D3, and just use the transistor, completely ignoring the "Opto" part of the circuit.

See the output circuit for the AVR335 appnote linked in my other answer for a circuit which effectively drives a speaker with a PWM. The filters smooth the output signal to something better approximating the input (With a rolling average), so that you get a smooth wave rather than a rough digital square wave. You can remove the unary gain amplifier, that's just to remove any feedback from the microphone (Which you don't have).

You really want some filtering and amplification on the output -It'll sound absolutely terrible if you don't. You know how those talking greeting cards sound? They have filtered outputs. Your speaker will sound worse than that if you just connect it to the PWM. I was previously assuming that you were using the phototransistor to isolate your output and transfer it to an amplifier/filter circuit running on a different power supply, but a straight connection is going to sound really bad.

The LED in the optocoupler is an infrared type rather than a visible type. As such, it has a much lower forward drop than even a red LED, so when the two are in parallel, the IR LED in the optocoupler hogs all the current and has a forward voltage drop of perhaps 1.1 or 1.2V, which is not enough to allow the visible LED to light.

When two similar visible LEDs are in parallel, they share current better, and will typically light to (visually) similar brightness. When the two LEDs are in series (and there is enough voltage for both) then the same current flows through both LEDs and both will light.

Here is what typical curves look like (this one for a Lite-on 4N35 optocoupler)

This one for Cree LEDs of various colors :