While possessing an entity in Spectator mode, the volume of sounds will be calculated from the position the player was in when they took possession.

Thus, sounds near to the players original position will continue at the same volume right up until the possessed entity exits the range of hearing, and sounds near the possessed entity's new position will be quieter and quieter until they are completely inaudible.

On a related note, as described in MC-46445, attempting to turn your head while possessing an entity will not have any effect on the entity's direction, but will change the direction from which sounds are heard.

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To reproduce this bug, use the setup shown in the screenshot. The noteblocks should be far away enough from each other that neither can be heard from next to the other. They should also have different enough sounds to be distinguishable from each other. Press the button, then quickly switch to spectator mode and possess the mob in the minecart. You will be able to hear the first noteblock at full volume for most of the trip, and the second noteblock not at all until you depossess the mob.