The first ability of Voice of the Blessed triggers just once for each life-gaining event, no matter how much life was gained.

If Voice of the Blessed is dealt lethal damage at the same time that you gain life, it won't receive a counter from its ability in time to save it.

Each creature with lifelink dealing combat damage causes a separate life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time, Voice of the Blessed's first ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.

If you gain an amount of life “for each” of something, that life is gained as one event and the ability of Voice of the Blessed triggers only once.

In a Two-Headed Giant game, life gained by your teammate won't cause the first ability to trigger, even though it caused your team's life total to increase.

Gaining flying after it has become blocked won't cause Voice of the Blessed to become unblocked.