Firstly I needed a good two tone source to use. Elecraft have a set of simple kits and one of them is a 2 Tone Test Oscillator. I like building my own equipment, so I got that.
Testing the audio is a bit tricky. You need to start with the right level for the audio source. If the audio source is set too high then we get distortion. You can back off the gain in the audio stages to remove the distortion, but then little RF energy is generated when the microphone is substituted for the test source.
I measured the audio level from the microphone with the oscilloscope. It is only about 40mV pk-pk and jumps about like crazy. I wanted the peak value to correspond with me speaking loudly, rather than a loud whistle, which is not really representative of normal audio. After a bit of frustration I managed to settle on 40mV pk-pk as the value to use. I don't really know if that is the right value though. I played around with this for quite a while.
I adjusted the 2 Tone Oscillator to a similar level then connected it to the radio with a patch lead. I attached a dummy load made from 20 1k half watt resistors and clipped the oscilloscope probe to one of the resistor leads.
Maybe the tricky part was not behind me, as I had hoped. I found it really hard to adjust the audio gain to prevent distortion. I seemed to have either no RF output or it was distorted. I resorted to checking the output at each stage of the transmit chain to find the problem. It turned out that the PA was no longer biased into Class AB (or A), but was now operating in Class C. Yikes, that means it is operating as a switch and is very non linear. That would certainly cause the distortion. I adjusted the bias potentiometer until I had a reliable clean signal. Normally I adjust the bias by measuring the current drawn by the FET, but this seemed better.
Success! I have not had any more poor audio reports since making this change. I'm not sure why the bias was setup wrong. I had adjusted this carefully when the radio was built. I also got many good clean audio reports. Perhaps I set it right at the threshold between Class AB and C in an attempt at keeping the current draw low. I am now using a different power supply and a slight change in voltage levels was probably enough to change the conditions.
Oh the joys of home made equipment...
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