Very many thanks for the kind and helpful comments. Your first point shows where I had rushed to premature judgement (it's the first time I've used the Dynamic Link though I've previously made scores of DVDs with Encore). Indeed the problem must be with something other than the dynamic link.
Yes, I exported the file from PP with MPEG2-DVD export settings (had picked that possible source of problems on another thread).
The source footage is 720x576 (1.0940) 25.00 fps (PAL-B) with 48kHz stereo sound. I just realized the sequence aspect ratio is 1.4587 (hangover from previous work): can that affect the sound ouptut?
You have convinced me my irritation with the Dynamic Link is probably unjustified. Creating a brand new project and importing the mpeg and ac3 files still produces a silent sound track. Will follow up your suggestions in the links.