Locus Sonus Stream Project offers a worldwide network of "open mikes" that permanently stream local soundscapes to a dedicated server. The resulting live audio is used in a large variety of artistic projects. The microphones are installed and maintained by volunteer participants.
To set up a permanent stream we suggest that you use our OS distribution for Raspberry Pi (mini computer costing around 30€) + a usb sound card and microphone. The PI is silent and uses minimal electricity.
Another option is to use this Pure Data patch on your home computer (running on Pd-extended).