If these symptoms are appearing on only a few of the older (lower) leaves, then it is just natural aging as the older leaves are replaced by healthy new leaves. If it is more extensive than that, then continue below.

The best light is bright indirect light. That would be right in front of an unobstructed, uncurtained north window. It might also be in front of a south window that is covered with the sheerest of curtains. Or it could be right under overhead office fluorescent lights. Any window with the shades drawn will not provide adequate light. Any direct sunlight is too strong for peace lilies.

It is best to water your peace lily just BEFORE it starts to wilt. Although peace lilies do perk up after watering, wilting does cause leaves to yellow several days later. Soil that is constantly wet will cause leaf tips to turn black.

Fertilizer and hard water both contain mineral salts. These minerals build up over time, burn tender roots, and cause leaf margins to discolor. The solution is to stop fertilizing (fertilizer doesn't help much anyway) and to use filtered or distilled water if your local tap water is on the hard side.

Sometimes it is just easier to trim off the discolored leaf edges when they appear and not worry about it. These symptoms are almost never a sign of disease or poor plant health.