Add some of your own sounds, such as a water feature or some stereo speakers, to help create your own ambience and drown out the world outside. If you have a larger yard or property and neighbors farther away or a source of noise like a road, it will be easier to block out unwanted noises. You can use the methods I suggested above close to home but add other features further away from your garden and nearer the noise source. Combine this with a landscape berm or mound to screen your garden from the road both visually and audibly. A good example of how this can be achieved can be seen at the Montreal Botanic Garden in Canada.
Internal sounds are another source of noise.
I'm referring to those from within your property. If you want a peaceful garden, one of the first things to do is to look at how much power equipment you have, the maintenance on it, and how much you use it. This past year while visiting gardens in England, I was struck by the lack of a lot of power equipment, including blowers and trimmers at most gardens. How nice and peaceful and quiet they were. For desirable garden sounds, first consider preserving and enhancing those you have, such as a babbling brook.