Since MysticCat said everything I was going to say, I'll just answer tfj-1893's questions in regard to Delta Omicron.

Much like Phi Mu Alpha Sinfonia, we willingly accept and actively recruit non-music majors and minors. I myself am one of those "recruits".... I don't have one iota of musical talent. Had I not been rushed by DO first, I would have joined an NPC on my campus. We run (or ran, at my time, things have changed) very, very much like an NPC sorority.

We have risk management programs, a national foundation, weekly meetings, ritual, philanthropy, socials, mixers, etc. When I was in college, we regularly held activities with the other "social" fraternities and sororities. We have formals and retreats. Some chapters have houses. The only way we differ from a strictly social sorority is that we have a "focus", as MysticCat put it, in music.

Edited to add: I thought I should note that as MysticCat mentioned, the variety in 'temperment' so to speak of these groups is wide, and can change very fast. Some chapters of DO, PMA, SAI, etc. are very social (like the chapter I pledged). Some are strictly professional. This depends greatly on the members at the time and what they want out of it. When I was an active, we were more interested in the social and sisterhood aspects rather than just having something to put on a resume. Five years later, a different group is in there and wants it to mainly be another society to boast about. In another 5 years, who knows, perhaps it will be a social organization again. It's hard to make a blanket statement about what a group is about when you have tens of thousands of members who may all want different things. Delta Omicron strives to be all of those things for every member.