I can't help you with Unices.

As for audio stream broadcasting you may use freeware streaming server Icecast. I don't think you will have any problem finding streaming players for it.

Icecast supports AAC. I would use 192 kbps AAC or MP3 with higher bit-rates. However I don't know anything about your music, so you may just test several bit-rates.

If I'm not mistaken 100Mbps LAN can be used to broadcast HD video. But it really depends on how your network is built. 1000Mbps network cards are not expensive so they are highly recommended.

To calculate bandwidth you can multiply bit-rate by time in seconds. 'coz you have multiple clients, you should multiply the given amount by number of clients.