Ok I deleted the c130 & SU57 mods from my aircraft mods folder, I deleted then re downloaded the the two source code files from github & unzipped them into the aircraft mods folder, restarted DCS & then ran the mission again.
Again the mission briefing commentary stopped after closing the canopy & I exited to desktop
logs are included below.
These logs are for the last trial I did in which I put the tar.gz folder inside the .zip source code folder after extracting them. I previously ran the mission with just the .zip source code folder in the aircraft mods folder & with both the .zip folder & the tar.gz folder in the aircraft mods folder, but in all three cases the result was the same.
thanks for your help
debrief.log
dcs.log