How do I find the end of speaker wire in a finished wall?
Well, the way I'd find it would be to get a wire tracer. These consist of a transmitter that hooks to one end of the wire to use it as an antenna, and a short-range (and sometimes directional) receiver which you can then use to find the wire emitting that signal. That will let you trace a wire through the wall's surface. (There are several variants of this, at different price points; the more expensive ones can even be used on live AC circuits.) 

[Originally a comment, since I thought it was a bit short/shallow as an answer.]