Daisy book on ipad via Dropbox

Hello,

I have been trying to transfer RNIB Daisy Talking book from USB memory stick to Voice Dream Reader (VDR) on my ipad via Dropbox.
On my pc I have copied the book into zip file, then put into Dropbox. I can get the files on to VDR via Dropbox, but they are blank with no sound. Help! What am I missing? I have transferred mp3 files, from other sources, in the same way and sucessfully.

Thanks for advice
Mike