I should think that most music on piano rolls are in the public domain
(anything pre-1923 in the US).  There are claims by the Harry Fox
Agency, which was made to protect earlier works on sheet music or
the player piano or phonograph, but it never takes down videos as far
as I know.

Being a remix artist, I know all too well about the minefields of
copyright law.  Also, I would imagine that someone shooting a video
of a baby dancing to Beyoncé, or a person mixing up a celebrity's or
character's words to say other things, or someone showing off their new
piano, are not intending to "steal" a copyrighted work or offer it for
free, but merely share what they love and/or have fun with something.
In fact, it can help promote the original work, should it become
popular enough.

What Rowland Lee says is quite correct, however.  Artists who do
make money off of copyrights don't usually make a lot.  In the record
industry, most artists get a measly 13%.  The labels (the ones who do
the lobbying, mind you) get a good 63% of all the profits made from
a song.  Distributors (the people who promote and/or sell the songs on
CD/MP3/LP) get 24%.  The 13% the musicians receive is then divided up
between band members (if it is a band), with each member getting about
18%, and the rest going to lawyers, producers, managers, etc.

All in all, a musician can expect to earn about $23 for every $1000
worth of music sold.  Even big-name stars like Rihanna have been
cheated this way, with her manager taking an extra large 15% instead
of the traditional 5%.  All of this, of course, pertains to bands who
are signed by a large record label and have a reasonably successful
song/album, not YouTube hits or independent labels.

In all fairness, I would much rather deal with copyright claims in
a more civilized manner instead of legal threats.  That way, I can
understand where they're coming from, and why they want it gone,
instead of being harassed by some faceless corporation.  The Internet
is vastly different beast compared to the real world.  If a person or
corporation throws a hissy-fit over a copyright, they may well be
poking a sleeping bear.

My 25 cents.

John Belmont