Working!
What changed?
Firstly, I was feeding the audio via an isolation transformer.
Normally good practice, but obviously rolling off the higher AF frequencies too much.
Remember, we are using the wider bandwidth 9600 packet data output, not the normal voice bandwidth AF output.
Secondly, not quite enough level.
Feeding the mic input, with level control suitable adjusted, works 100%.
There was not quite enough level coming out of the, in this case, IC-R1500 to drive the sound card's line level input.
