Hi again Gordon,
I downloaded the musicplayer Flash button from Stan Bogdanov's site on his Flash mp3 players page. I made an exercise which validates and works fine outside of Moodle. It also works fine on Moodle 1.9.19 in QuizPort as you can see here: 1-9 Audio. It also works in Book on 1.9.19.
But on Moodle 2.4 it doesn't as you can see here in TaskChain: 1-9 Audio - it doesn't work in Book either.