Make transposed tunes use the original audio. The aim of MIDI and MP3 is to allow the user to hear what a tune sounds like. For transposed bookes, I think this should be reused from the main booke page, because hearing it at the transposed pitch isn't a major help. And in fact given the way cello transposition is done, the sound won't be transposed anyway. And Jane hasn't complained. This will speed up building the bookes by removing audio generation, the slowest process, from the transposed bookes.

% -----

\newpage
\section{List of tune first lines}

You know the feeling. Everyone has just launched into a well-loved tune.
You've heard it before, but don't remember the name. So here's a list of
tune first lines to help you figure out what it is in time to join in.

\setlength\LTleft{0pt}
\setlength\LTright{0pt}
\begin{longtable}{lr}