The non-draining pot is the first red flag, although with Spathiphyllum it isn't as much of an issue as it would be with most houseplants. Next would be the MG potting soil, Many people grow plants fine in it but there's also many who have problems because of it. It is very peaty and holds a lot of water, then when you let it dry out it becomes a hard block.

Roots need access to air, they absorb oxygen from the air and when there is too much water in the media the roots are block from the air that is between the tiny spaces between the soil particals. When you let the soil dry so much the the plant droops or wilts this type of peat based mix dries up so hard that it also can block enough air from getting to the roots.

You'll need to find out what damage has been done to the roots. If you un-pot the plant and wash the soil off and the roots are soft and brown it was most likely the soil issue I've discribed above and you'll need to rethink how you grow this plant. If the roots are firm and white(ish) then you were probably waiting too long between watering. Spaths also drop an occational leave

Spaths can thrive in bog like conditions that many other plants will drop dead in. In nature they can often be found growing in bogs and swampy low hollows in the tropical forests.

If you want to continue using the same non-draining pot you should switch the mix you're growing in or change the whole thing over to passive-hydroponics.

Spathiphyllums are essentially bog plants. Letting them dry out is very bad for them. They're from high rainfall environments so their water is highly oxygenated, unlike tap water which is usually the opposite. So too dry as well as too soggy with oxygen-less water will cause big problems. If you're using tap water your soil needs to be light, loose and fast draining to allow the air through to supply oxygen. But never let the soil get to the stage of becoming dry.