1.) Are there any pointers to URLs that deal with issues to set up for both incoming and outgoing multicasts? I can't find many examples that show this from an ISP point of view.
The best tutorial IP multicast training materials I'm aware of are those at ftp://ftpeng.cisco.com/ipmulticast/training/index.html If you're looking for something more cookbook, I think the Internet2/Abilene multicast cookbook at http://www.abilene.iu.edu/mccook.html is also a nice resource (although obviously it is most useful if you are connecting via Internet2/Abilene).
2.) Is it worth my effort? In other words, what % of the internet audience is capable of receiving multicast IP from an audio feed?
One data point is that there are over 4600 MBGP prefixes getting advertised (see, for example Marshall Eubanks' presentation at http://www.nanog.org/mtg-0110/ppt/eubanks/sld011.htm). Whether or not that's enough of an audience to make IP multicast worthwhile is a decision you'll have to make for yourself. But if nothing else, by enabling IP multicast, you should be able to receive technical content such as the NANOG meetings, IETF meetings, etc. (which I would assert is worthwhile in and of itself).
3.) Are there any examples of audio feeds that use Multicast I can test with?
The University of Oregon makes KWAX (classical radio) available via IP multicast; if you are using IP/TV or a compatible client, select the session "UO Presents KWAX Classical Radio"), and we also have a variety of video sources up for folks to try. Regards, Joe