OMG I have the same issue for a year now. I even posted in the bug section but nothing. I will lose squads or units without any audio alerts.
I think everybody had this. Squad lost or grenade thrown at them? Sometimes no voice line.
I'm not too sure if this is a real bug or just the result of overlaying voice lines. Because some voice lines need to be prioritized over others, and it also might be that the priority settings have been done sloppily. So less important voicelinrs could be prioritized over more important ones.
Still, there might be a bug component to it.