I canned mushrooms with fruit fresh. They have a very acidic taste - tart not sweet. Can I fix them or make a soup from them to salvage the batch?
Fruit Fresh is a brand name for citric acid. It's used in canning to maintain an acidic environment to prevent spoilage. It's name comes from the fact that it's the same acid that makes citrus fruit sour/tangy.
It sounds like you've made some kind of pickled mushroom by canning them in an acidic environment. Some people quite like picked mushrooms, and if you search the web, you'll find many recipes for them.
Generally speaking, if something is too acidic by itself, you can balance it out with some "opposites" by countering with sweet (like adding sugar to lemon to make lemonade), fat (like mixing oil and vinegar for a salad dressing), even spice (like a hot and sour soup).
As for how to use your accidental pickles, you might start by looking at recipes to make pickled mushrooms to find one that appeals to you, and consider if there's anything more you could/should add to round out the pickle itself (ie, herbs, spices, etc). This may help you turn your accidental pickle into a palatable pickle. Then, secondarily search for a recipe that specifically uses pickled mushrooms.