PWM + resistors = very short range (4 notes?), notes sounding properly (switching immediately)
I tried for about 2 hours plenty of combinations of resistors in series, parallel, even with capacitors somewhere there... Both between the PWM and the inverter input, and between the inverter's input and output (diagram attached).
It sounds like you are trying to make an "old" school synthesizer with digital controls.
Total waste of time [...] any form of filtering is just going to get zapped out of existence by the DIGITAL inverter. [...] The PWM when filtered can give you an analogue voltage level, you need then to apply that to a voltage controlled oscillator.
My VCO is the hex inverter