live audio webcasting

Being my favorite site with the best advice, I'm going to go totally stupid and ask for recommendations on the topic of streaming audio about which I have had zero experience! Mission is:
live stream audio from 3 or 4 "conference room" meetings monthly, lasting about 7 hours, for anywhere from 2 to 50 people. Ability to sometimes add a "secure" or password-login protected cast required occasionally...
Hey, need microphone inputs to a sound card, a streaming server and an Internet connection. Fine... There's Shoutcast, etc... Why would I want to host this myself, as opposed to hosting it remotely? Yeah, think firewall.. Don't know? I can provide dedicated T1 bandwidth (but how many listeners will it cover?) We're a state program, that among other services, helps deaf and disabled people get free telecommunications equipment in California to let them communicate; the purpose of this meeting audio is to allow folks who can't make it to the meeting listen in.
Any logical advice much appreciated!
zakrz1