iSight FireWire camera microphone not detected by Siri on Mac mini

If you were running Ventura or later, my first guess would be that Apple had removed drivers from macOS that the iSight camera relied upon. There have been reports from people using FireWire-based audio gear of Apple removing support for "plug-and-play" FireWire audio gear that doesn't have its own drivers. (And many separate drivers for FireWire audio gear would likely be in the form of Intel-only kernel extensions that wouldn't run on an Apple Silicon Mac.)

However, since you are running Mojave, that seems unlikely to be it.

You say you "Have iris closed for privacy." Have you considered the possibility that this might also be disabling the microphone for privacy? What happens if you try to use the microphone with Siri with the iris open?

I recall that the Apple iSight camera also gets plenty hot and what you may be experiencing after all these years is the beginning of device failure.

Has Siri ever worked with audio on this camera?