The peace lily is a moisture loving plant; it may not be getting enough humidity because of the warmer summer temperatures. Try getting a spritzer bottle and spraying it a few times a week. For watering, make sure you are completely saturating the soil when you water, but never let your peace lily stand in water. Before watering again, use your finger to test the soil to make sure it feels dry to the touch.

When you changed the soil, did your roots look healthy? A healthy peace lily should have white roots. If your roots were not white, your could have a problem with the root system.

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However if it is a new flower, you need to check a few things. Moisture level, light exposure and insect infestation. The plant needs to be moist but not soggy. If the plant was too dry at the time the bloom formed, it could cause the browning. If the plant is receiving too much light, the blooms could be scorching. Finally check for insects. They could also cause the bloom to turn brown.

Once you find the culprit and correct the issue, the new blooms should be fine. I recommend trimming the damage blooms off.

I think frost damage could be possible. In the winter, my mother's windows tend to let in a bit of a draft. If your window is not insulted and you can feel a draft, then the peace lily definitely was exposed to to much cold. Move the peace lily a little farther from the window and make sure it is out of any draft flow. Do trim the black leaves back to the base of the plant. If you haven't fertilized the plant lately, give it a very diluted amount of liquid soluble fertilizer. Also, check for root rot – make sure the plant is properly draining. You may need to re-pot the peace lily; if it isn't draining properly.