These flags only apply to monsters that can do idle talk, like scientists. Human Grunts can't do it in vanilla HL. For programmers, any monster that inherits from CTalkMonster supports these.
If the Concurrent flag is
NOT set, then multiple monsters can talk. If it's set, other monsters will be silenced.
The Interrupt Speech flag will allow monsters to talk even if their normal conditions would prevent this. For instance, if other monsters are talking, then this flag will ignore that and allow the targeted monster to talk as well. This only works if the monster is alive.
If you use Interrupt Speech with Concurrent, the latest sentence takes priority. All other talking monsters are silenced.
If Concurrent is
NOT set and Interrupt Speech isn't set, then the monster might not speak at all if another monster is speaking due to condition checks.
If Interrupt Speech is set, and Concurrent isn't, then you can let any monster talk at any time without being interrupted.
Setting the Gag flag and not setting Interrupt Speech will cause monsters to never speak any sentences.
I know this is confusing. The Concurrent flag actually disables concurrent speech. This is why:
https://github.com/ValveSoftware/halflife/blob/master/dlls/scripted.cpp#L1171The flag's state is inverted. So setting it disables concurrent speech, rather than enabling it.
If setting Interrupt Speech and not setting Concurrent doesn't work, then something else must be wrong.
Is the Followers Only flag set? If so, the scientist will only say something if they're following a player.
Try enabling developer mode. Set developer to 2 or higher to see sentence information in the console. That should help you identify the cause of any problems.