Ok, sorry for thread necro, i know its a year later, but as there were more people with this issue and as latest stable release (3.7.4) still has the issue, i wanted to post the following:
1) The issue is NOT present in 3.8.0 RC3
(can't comment about any builds between 3.7.4 and 3.8.0 RC3, as i didn't test in them)
2) Possible sources in 3.7.4:
2a) issue might
be related to EFX effects. As far as i can tell, it only presents in dialogue lines which use EFX to enchance them. This may be completely wrong, as the only way i could "measure" that was to enable/disable EFX in 3.8.0 in launcher, launch mission and see if i can hear a difference in dialogue sound. It seems to affect Kassim lines the most? The peculiar thing is, disabling/enabling EFX has no effect on issue in 3.7.4, so i may be completely off here.
2b) issue might
be related to Laporte's "thoughts" (i think BP uses training/instructor box for that?) - it might be subjective, but i think dialogue lines that came directly after one of those appearing where sometimes quiet.
2c) issue might be related to dialogue lines being interrupted with other dialogue - i think dialogues that cut off one of "generic" pilot lines were sometimes quiet.
3) As the issue seems to be resolved in 3.8.0, fell free to disregard and bury this thread