I think I have found what is causing the issue - I have attached a save from another mission. If you select Ares (soldier 5 in the list) and follow the path shown in this pic, as soon as it breaks through the barrier (circled in yellow) it reliably reproduces the sound issue. Interestingly, it doesn't happen if you don't break the barrier. Nor does it happen if you go through the door and stop before hitting the barrier, then go through the barrier. It seems to only happen if you go through a door THEN go through some sort of obstacle. Also, once it is triggered, even if you reload the attached save without quitting first, the sound error loads too, even though you have moved the Ares back to the start.
output.log
output.log auto_groundcombat_turn_5_start-364.json