My experience with Peace Lily, they prefer dappled light to shade, and heat really takes its toll on it. But it is a very vibrant plant, I have one which was to all intent so dead after being left outdoors in 100+F heat during summer last year. It was on the shadiest side outdoors but the heat is just too much for it, plus maybe my hubby forgot to water it too at times:
Photo taken on 08Sept2011
After seeing that, I felt inclined to end its agony and just toss it out, but I still see something green, so I just gave it a real good watering, keeping it outdoors still on the same spot, hoping that the incoming cooler Fall Season will help it revive. Just being mindful of keeping it watered when it is semi-dry.
By 18Oct2011, the plant has formed new leaves, it is now gradually recovering:
As the Fall season ended, I have moved this plant indoors so it can winter indoors.
Now that it's Spring, it is still indoors, and this is the latest photo:I just tossed the blue water gel beads..just some excess I have from another project, it is just water anyways..
Now I hope it will try to make some flowers so I can really say it has fully recovered
As with what the others had said about the sunburn, that is plausible, plus from my other experience with other plants oftentimes the plants will pout when it senses temperature/light variances, and will show it with its leaves slowly turning black. But it will adapt and adjust eventually. Now just do not make the mistake of overwatering too, just water enough to keep it moist.