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.

%
% Take the A5 landscape dottes.pdf and produce a second PDF
% formatted for A4 portrait, which two pages per A4 page,
% and ordered for booklet printing.
%
% Print the resulting PDF with long side duplex.
%
\documentclass[a4paper,final]{article}
\usepackage{pdfpages}
\usepackage{ifpdf,ifxetex}

\begin{document}
\includepdf[pages=-, nup=1x2, booklet=true]{dottesA5.pdf}
\end{document}