I'd suggest you get all new soil & cut off any dead growth.

How about a picture first?

You'll need to pls. to repot w/ ALL NEW MIX & not add back those 'amendments' (which are not helpful for INDOOR containers).

It's generally better to stabilize a plant before starting to add things to its mix.

Perhaps also try reading other posts here about PL (Spathyphyllum) to see &/or try to learn which mix would be more appropriate for your plant.

"As I was doing this it seemed like the roots were moldy, since they had a white, fuzzy layer on them,"

What you saw is likely not mold-it's probably just the root hairs of the root. Those little hairs do the "lion's share of the work" (phrase commonly used in gardenweb) when it comes to taking up water and nutrients.

"So I just placed it into a new pot and put a little bit of organic fertilized (ground egg shells and coffee grounds) "

About coffee grounds-I researched a little on gardenweb to see what other people say about using them as fertilizer.

" coffee grounds will break down into fine, small particles that will impede drainage and suffocate roots." (http://forums2.gardenweb.com/discussions/1487604/coffee-frequency-question?n=8)

"There are many more good reasons to avoid using coffee, tea, and coffee grounds in container culture than to include them." (same topic)

Then again, I did find people who say that they throw coffee grounds in their pots and their plants always bloom each year. I'm not sure-whichever you think is best. Personally I wouldn't use them-I would just use a 3-1-2 fertilizer (MG 12-4-8 or 24-8-16. or Foliage Pro 9-3-6) .It gives a much bigger dose of nutrients than a few coffee ground sprinkled on top.

Now to the issue of watering:

"when I got back half of the flowers were dark brown and the leaves were withered to the table."

Flowers will all eventually die-just because a flower is turning brown and dry doesn't mean the entire plant is in jeopardy. However-the leaves drooping to the table tells me that the plant itself needs water. However, that doesn't mean it's underwatered. When overwatered too much, the root hairs than I mentioned above begin dying. Without the hairs, the plant begins to have trouble taking up water and shows signs of thirst above, which causes the owner to water more and more. Essentially the plant is dying of thirst in a sea of plenty-there's water all around, but the roots cannot easily take it all up because the fine roots have all died.

What to do? I would unpot it carefully and check the roots for rot. Healthy spathiphyllum roots should be white or tan, and rotten ones should be either black or dark brown, and they should smell horrible. If there is root rot, then I would rinse off all the old soil, sterilize a pair of blades or pruners with rubbing alcohol, then cut off all the dead roots. Afterwards, I would pot the remaining root mass with a 50/50 mix of potting soil and perlite (or 5-1-1).

However if the soil mass is completely dry, that means that the soil has become water repelling, which means each time you watered the water just flowed down the sides of the pot and out the drainage hole, completely missing the roots. If that is the case, I would put the plant back in the pot, submerge the entire pot in a bucket of water and wait for it to stop bubbling. Afterwards, take the pot out of the water and allow it to drain before putting it back in its original spot.

In the future, you can figure out when to water by using a wooden dowel or chopstick. Insert it all the way into the soil, leave it for a few seconds, and pull it out. If it is dark colored/obviously wet, then withhold watering. If it is dry, then water.

Before you follow any advice, would you mind posting some pictures of the plant? They would be extremely helpful. Sorry about the long post!

I hope your plant recovers well!