Hi Ginger.
My mom found this cookbook for me, Secrets of Lactose-Free Cooking by
Arlene Burlant, RD... it is published by Avery and was on sale in one of
our local drugstores. It has given me quite a few ideas for recipes for my
milk protien allergy daughter and one of the recipes I found especially
interesting was the base for Quick Cream Soups. Here is the recipe:

2 Tablespoons of Non-Dairy Margerine
6 Tablespoons of All-Purpose Flour
6 Cups of Reduced-Fat Soymilk
and Black Pepper to taste

In a large saucepan, melt margerine over low heat. Add the flour and stir
until smooth. Remove the pan from heat and add the soymilk. Return the pan
to the heat and bring the mixture to a boil. Lower heat, and simmer,
stirring slowly and constantly for about 10 minutes. Add pepper and stir.
Add 2 to 4 cups of cooked, chopped vegetables, poultry, or fish. Continue
stirring to heat and stir as necessary. Serve Hot.

I am sure you would be able to just add mushrooms to this recipe for a
mushroom soup.
Good luck and let me know how you like it.