Talk over PowerPoint audio

I'd recommend mixing the PPT audio and your mic into a single feed via something like Voicemeeter (https://vb-audio.com/Voicemeeter/), and THEN setting your audio input to 'Original Sound for Musicians'.

In Voicemeeter, you'd bring in your hardware mic input and a virtual input (after setting your default output for PPT to Voicemeeter virtual input) and then send these out Voicemeeter's virtual output.

After that, you'd select Voicemeeter's virtual output as Zoom's mic input and ensure that original sound for musicians is selected.

When sharing the PPT, do not share sound, as this will be passing through your mic input already.

Have never had an issue doing it this way, but the method is a bit convoluted if you're not used to working with digital audio.