When the Captain's navigation announcement came on, was the in-cabin TV turned on and tuned to the Navigation channel (which also plays music)? If yes, then that is the reason for the announcement to be heard in the cabin. The other speakers in the cabin are used exclusively for "emergency" announcements from the captain (e.g., fire, fire drills, crew emergency drills, medical emergency, weather emergency, etc.). The navigation announcement can also be heard throughout the ship thru the address system in the public areas of the ship, including speakers in the cabin hallways. if your cabin door is open, even slightly, the announcement will be heard in the cabin.
VFR