If the leaves are wilting, then the plant is not getting enough water. I can think of three reasons this might happen.

1) You're not watering enough (which, according to your info, is not the case).

2) The plant cannot take up water due to salt accumulation in the soil from fertilizer/tap water that isn't allowed to drain properly.

3) The soil is holding too much water causing the roots to rot.

If the problem is salt accumulation, then you need to water the plant enough that a good portion of water runs out the bottom of the pot. This flushes out excess salt and should be done as frequently as possible. Also, don't allow water to sit in a dish underneath the pot.

And I know you said the roots looked healthy, but a rotting root ball can still have some healthy looking roots near the surface. I'm including a link to a post I made on another thread about a peace lily. In it I show pictures of roots from a rotting peace lily and a recovered one. You can see that there are still some firm, white roots on the rotting one even though it was wasting away. A healthy root ball should look like the one in the second picture. I've found that my plants do best when peat is used very sparingly, if at all.

Here is a link that might be useful: Sad Spathiphyllum

When you water it, does the pot feel considerably heavier? Sometimes potting mixes cant absorb moisture if it is allowed to dry out too much.