In playing the second mission, after defeating all the critical targets in the middle area (in the fourth, and I assume, final phase in that area), I think I got one or two more voicelines from that area indicating continued combat before all voicelines cut out completely, including AWACS callouts, lines related to other operations, etc. I received no indication that the operation in that area was successful other than that no more critical enemies appeared. When I went to the West area, the operation there proceeded as normal, but still without any voicelines playing. Neither the audio nor the subtitles appeared. All other audio (music, SFX) continued working like normal. After restarting the mission, voicelines were working again. I suspect I defeated all the critical enemies before the scripted voicelines for that phase finished playing and that caused the bug.
I also encountered the same bug in the Dust Lance operation. On the final airfield assault, all targets are destroyed, but then no further voice lines are given and the airfield is not marked as "friendly" like it should when that operation completes. I have completed that operation before, and didn't see any of the usual indications that it "completed" this time.
One thing I did notice is that the last red marked enemy tank was destroyed by an ally. I do not know if that was the cause - I will need to try the mission out a few more times to see if I can replicate this.
