sefirat ha omer - Why should A Capella music be allowed during Sefirah/Three Weeks?

Background:

I'm not sure how I feel about: a)the custom to refrain from listening to recorded music during Sefirah or b)the assumption that recorded a cappella music is not considered instrumental (despite the fact that there is nobody sitting in front of you singing; ie., the recording is itself an instrument). OK, I'm pretty sure we all can tell how I feel about b. Anyone disagree, though? And any thoughts on a?

I'm also not entirely sure why live singing isn't a problem during Sefirah (assuming it's done for the sake of making people happy and spreading joy).

I understand that refraining from listening to music appears to be Minhag Yisrael, but it's not mentioned anywhere before later Aḥaronim - even the RaM"A doesn't mention it: http://www.hebrewbooks.org/pdfpager.aspx?req=9008&st&pgnum=292 (or does someone else contemporaneous to or preceding the MeḤaber mention it?).

To be clear, I'm operating on a couple of assumptions, so please correct me if I'm wrong:

1. The practice to refrain from listening to music during the period - not discussing the prohibition the rest of the year, which is clearly not followed by most - is a (comparatively) recent trend.


2. I don't believe the prohibition was ever an issue of personal joy, so much as public expressions of joy/celebration.



3. Live performance of music is one of many forms of expression/manifestation of public joy and celebration, especially when accompanied by dancing.

So, considering all of the above, why is live a cappella ok? And if instrumental music itself is a problem why is recorded a cappella ok?