MES
Enthusiast
I've used NGK plugs for 10k miles now (2 sets at 5k each) and had no problems. I've never noticed a sound change in the exhaust over stock plugs. If you do have a quieter car maybe one of your old plugs were going bad
I wouldn't be concerned about the noise reduction as long as the car is running good.
Edit: I'm using ZFR6F-11 #4291 not FR5-1 I think the 5 is stock and 6 is one step colder (but could be wrong)

Edit: I'm using ZFR6F-11 #4291 not FR5-1 I think the 5 is stock and 6 is one step colder (but could be wrong)