In theory, correctly filtered PWM or PDM at a correct modulation rate is identical to the original audio (nyquist-shannon). In practice the square waves produce

In theory, correctly filtered PWM or PDM at a correct modulation rate is identical to the original audio (nyquist-shannon). In practice the square waves produced have so much harmonic content that you need a good filter (like the active opamp filter in the article), and the simple RC filter that a lot of these projects use will sound terrible due to their long cutoff slope. Very high fidelity sigma delta DACs are filtered PDM, they just use high modulation rate and good filters.