The failure of the drop-down/popup layer menus to work was my fault. I had done a lot of copying and pasting and applying repetition to the menu items in each of the 30 or so alphabetical song files and had obviously failed to apply the repetition in this case. The popup layer menu_L1 was present for every song but it took me a while to realise that all these layers were empty except for the first one.

Apologies to anyone whose time I've wasted but I don't think the original problem could be put down to the same reason. A popup would work sometimes and not others.

I've learnt some useful stuff from members on this thread so I'm off now to change all the audio files - I may be some time.

Many thanks for all the help.
Ian