You can generally have quiet and smooth, or you can have efficient. The settings you would use to get the marker as efficient as possible tend not to be the same settings as you would use to get the marker quiet and smooth.
For quiet and smooth, people will tend to increase their dwell and reduce the operating pressure - lower operating pressures aren't necessarily quieter as such, but the sound signature of the marker becomes less harsh. More of a pop than a bang. Problem is that you'll tend to use more air per shot this way.
For efficiency, people will likely reduce dwell to limit the amount of air released, but may need to increase operating pressure to get the ball up to speed, but your marker gets louder.
The middle ground would be to take your boards stock settings (rather than increasing or reducing dwell) and chrono your marker to speed. That is just to set your operating pressure. You can then do one of two things: -
1. See if you can reduce dwell without your velocity falling. It isn't uncommon for your marker to release more air than it actually needs to to get the ball up to speed. Lower dwell until the velocity drops, and then just bump it up a little until consistency is good and shoot down is eliminated.
If your dwell is now lower than the stock dwell, then you have just saved yourself some air. You have however made velocity consistency more reliant on dwell.
2. Do the same thing - chrono up to speed by adjusting your HPR - but now wrap a decent amount of tape in between the two front o-rings of your bolt pin; lets say to fill up about half of the space. Chrono again, and chances are that your velocity will have dropped. You can now start removing one layer of tape at a time until the marker chronos back up to the correct velocity.
This way, you have limited that amount of air released with each shot, without reducing the dwell setting, so your marker should be more stable in terms of velocity consistency and drop off, and may avoid bolt stick issues.
There are other things to consider of course - good paint to barrel fit will assist with efficiency, and a longer barrel with more porting will tend to help quieten your marker, lighter paintballs can accelerate up to speed with less air (though they will have less momentum than a heavier ball) etc.
Fact remains that the two things you are trying to achieve somewhat contradict each other, but if you try combinations of the things I have mentioned, you should be able to find a happy medium.