we have routers that you can monitor traffic on

Most traffic is encrypted with HTTPS unless you can root every single device you own

we have microphone use indicators on mobile, and I would imagine it would be pretty clear if an app was uploading audio with even very basic monitoring tools.

Complicated smartphone OS, firmware, drivers might have bugs allow overrides of visual indicators.

Companies have also been known to secretly eavesdrop and not tell users before (Apple + Siri https://www.courthousenews.com/judge-approves-95-million-app...)