you need to make a mod to do that. Either edit the player.entity or make a copy of all the advent voice dialogues and rename them to the vasari sounds. Entity is probably easiest as you just need find 'phase' (Vasari) and replace 'psi' (Advent) in the events section.
gameEventData
GameEventSound:AllianceCeaseFireBroken "EVENTPHASE_ALLIANCECEASEFIREBROKEN"
GameEventSound:AllianceCeaseFireFormed "EVENTPHASE_ALLIANCECEASEFIREFORMED"
etc