I have a few thoughts on this issue:
A. Heavier and quieter ammo could make a difference if it's between subsonic and ultrasonic. I.e. a light round at 1200 fps vs. a heavier round at 1000 fps. However, it will still be loud enough to easily damage your hearing.
B. With that in mind, you do not want to sacrifice effectiveness just to save your ears. It's your life vs. your hearing. If you can find a subsonic round that you're positive will be effective, go for it, but know that it probably won't save your hearing.
C. There are two proven ways to prevent/reduce damage to your hearing: 1. never fire a round in self-defense (which sort of contraindicates
>>654711
because you'll be lacking the situational awareness to avoid a gunfight in the first place); b. protect your hearing 100% of the time within reason (i.e. avoiding listening to loud music or going to concerts, wearing the best earpro available while practicing, etc.), which will help ensure you make a full recovery should you ever have to fire a round in self-defense.
D. I tend to think that in many situations, being able to hear is absolutely crucial in the moments leading up to the shooting. If someone breaks into your home, being able to hear exactly where he is could make the difference between you living and dying.
E. On a similar note, putting on ear protection can cost you valuable seconds and decision-making brain power. Even if you train frequently and get to the point where putting on your earpro becomes muscle memory, it's still one more thing you have to do, and if you fuck it up it'll throw you off. Then what happens if you don't wake up until the bad guy is right outside your door? Your muscle memory tells you to grab the earpro, but your brain tells you there's no time, so your OODA loop is fucked.
tl;dr just choose the most effective ammo you can, use earpro when you practice, but don't worry about saving your hearing when your life is on the line.