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.