The PC speaker didn't have PWM; it had a square-wave generator with adjustable frequency. Very few 1980s programs achieved anything even approaching decent sound quality using the PC speaker, although by the 90s, CPUs were fast enough that it was reasonably feasible. In the late 90s, I used to patch my Linux kernel with an unofficial driver that provided arbitrary PCM output via the PC speaker. It was glitchy if I left interrupts disabled during IDE disk accesses, which was the default at the time due to some buggy IDE devices that would sometimes corrupt data if you didn't. Fortunately, mine wasn't one of them.
I feel like PDM is just as simple as PWM and often gives better quality, and an R-2R DAC like a Covox is arguably just as simple as PWM and gives dramatically better quality. You need a few more pins, though, or a 74595.
I must be thinking of a different computer then. Perhaps my Radio Shack CoCo. I remember some small computer from the mid-late 80's producing really good music with just a logic output driving the speaker. Was sure it was the IBM/clone.
One of the nice things is that I have original cloth-bound IBM PC 5150 Technical Manuals in my basement so I can actually go look up the schematics of what's driving the speaker :-)
The PC was built to beep, and that's about it. Anything you got out of it past that was painful and used a lot of CPU.
While true, the quality of sound achievable this way is a matter of software. There are many programs that achieved excellent sound quality, playing music even with just a speaker and a TTL drive.
Remember, the simplest DAC is PWM into a low-pass filter.