Is "-6 to his stage points (to a minimum of 3) for the rest of the turn. " applied once and later changes of amount of stage points are added/subtracted to/from resulting value,Poison of his Voice wrote:Magic. Spirit-magic. Playable on a hazard permanent-event on a character in a spirit-magic-using character's company. Discard target hazard. Alternatively, playable on a spirit-magic-using Fallen-wizard. -6 to his stage points (to a minimum of 3) for the rest of the turn. Unless the spirit-magic-using character is a Ringwraith, he makes a corruption check modified by -3.
or
"-6 to his stage points (to a minimum of 3)" by the rest of turn is applied to an amount of stage points and the result is recalculated each time when the value changes?
Example of 1st variant:
Player has 4 SP. After playing Poison of his Voice he has 3 SP. If in the same turn he would play a card giving him 2 SP, he would have 5 SP.
Example of 2nd variant:
Player has 4 SP. After playing Poison of his Voice he has 3 SP. If in the same turn he would play a card giving him 2 SP, he would have 3 SP ["-6 to his stage points (to a minimum of 3)" is applied to the amount of SP before Poison of his Voice's effect and recalculated].