My only problem with this is the following: It doesn't appear to
allow a crescendo to happen throught a single note without some extra
hackery added. It would be better (IMHO) if we could emit
Audio_dynamic events (or Audio_tempo events) at discrete quantum
intervals. It doesn't seem possible, however, to emit Audio_items at
any moments other than those defined in the Music proper.
Suggestions? Maybe Audio_staff::audio_items_ can be a priority queue
(by moment), allowing us to emit events out of order?