Frankly, given the average 'clock' frequency for a PDM signal for audio frequencies, don't expect to generate the PDM signal directly from a standard PC without a specific interface.
Of course, that could be a simple job on a microcontroller if you can do that.
IMO, the simplest option is to build a very simple PD modulator like the one attached (first-order modulator). With a reasonably fast comparator, it will give pretty decent performance. Then you can feed it with any audio signal. Just a couple parts. This is a free-running ("clockless") modulator, so you have to figure out whether your demodulator will be OK with that. From simulation, with the given comparator, the average PDM frequency will be around 10MHz or so. You can use a slower comparator for a "slower" PDM output.
I've built this kind of modulators quite a few times in the past and they are very simple and work very well for transmitting audio with a decent quality with just one digital line.