What happens if I use fresh mushrooms to make vegetarian Chinese mushroom soup (冬菇湯)?

Fresh/raw mushrooms are quite bland by themselves (yes I know there are exceptions).

I suggest pan fry in oil them before using them in the soup, or chop them up and bake them in the oven on a sheet pan.

When you are using dried mushroom, you will re-hydrate them in water and you will usually use the resulting liquid which contains a lot of flavor in the recipe.