I wouldn't restrict the airflow to reduce noise.
With a 1/4HP blower, that is probably pulling like what - 10 Amps at 115VAC ? You will need a heavy-duty speed controller for industrial type use. I bet Granger would have something, or "Google it up" and see what you find. A speed controller could be made for most any fan, but the cost will begin to get up there as the amperage draws increase above 5-8 Amps or so...
Mine only pulls 3.5Amps (I think it is 1/25 HP), so a common 600 Watt/5 Amp fan control works for me.
The pulley and ratio-change would also work fantastic as the motor itself is not the "noise culprit". Slowing down the blower itself would do the trick.
Later