I was able to get the Mark Twain Project's  _Mark Twain and Music_ CD last
spring by writing directly to the Project.  I forget whether it was $12 or
$15, plus shipping, but anyone who is interested could write and see if
copies are still available.  It's worth the effort.  Robert Hirst's lecture
is  informative and entertaining, and the choral pieces are well sung.

And Camy -- Hirst himself sings a little of the "There Was an Old Horse and
 his name was Methuselah" song, then explains that we only have a few of the
lyrics, but apparently, the missing "bawdy" verses Twain sang were someth
ing to behold.  He wrote to Will Bowen that he "made that song famous in
Washoe" and bragged, "and didn't I make them solemn missionaries' eyes bug
out? I think so."  (I'm guessing at the punctuation; I'm listening to the
recording as I type.)