Must be these commands:
onBriefingGear sound
onBriefingGroup sound
onBriefingNotes sound
onBriefingPlan sound
Description:
Define sound (voice) played first time when section Plan in briefing is selected.
Example:
onBriefingPlan "PlanVoiceOver"
As far as I know the problem is that the sound is stopped/changed as soon as the player changes the briefing selection.