I have a gorgeous Peace Lily that was given to me about a year ago. I have just recently noticed (and it may have been there before) the brown area on some of the stems just before the leaf. I have no idea if this was always there or has just begun to show. Have you ever seen this before?
This is most likely just the natural aging of the plant stems.
If the plant is healthy and growing, I would not worry about it.
Here is a link to refresh you on the care requirements.