Cheap/Small Wireless Streaming Camera (V380)
Getting an ultra cheap wireless IP camera to put out an RTSP Stream with reflashed firmware. This enables it to be firewalled off the wider internet (local view only).
You can pay thousands for Bose multiroom audio systems, or buy a huge stereo and crank it up so you can hear it throughout the place. You can also get multiple digital distribution amplifiers that run over Wifi (eg Sonos), but again they come at a huge cost. What you really want though is cheap DIY multiroom background music system….