Agree with Tara. However, sometimes symptoms of over and underwatering affects the leaves similarly. So if it were my plant, I will check the soil & roots, just to be sure it is just moderately moist. Or sometimes the soil may be gradually clumping up, so it is slowly suffocating the roots, so slightly tease the soil, add some perlite, so there is some airflow there.
Peace Lily is very resilient. I had one that looks pathetically dead after being left outdoors in summer, but it gradually recovered with careful nurturing in the shade and kept moist.
