Place the dried mushrooms in a bowl or large measuring cup and cover with about 1 cup of hot water. Let the mushrooms sit and soften for about 15 minutes.
While the dried mushrooms soak, cut the vegetables. Slice the mushrooms and carrots, dice the celery and onion, and mince the garlic. Place the vegetables into a stock pot with the olive oil.
When the dried mushrooms are softened, strain them in a strainer lined with a coffee filter, reserving the liquid. Rinse the strained mushrooms well in cold water to remove any sand or grit.
Mince the softened Porcini mushrooms and add them to the vegetables in the stockpot.
Heat the stockpot over high heat, and saute the vegetables for about 12 minutes, or until they wilt and turn light golden and any liquid from the mushrooms has evaporated. Stir frequently and reduce the heat to medium if the vegetables start to burn.
Add the reserved mushroom liquid and the vegetable stock to the stockpot. Then, add the thyme and salt.
Turn the heat to high, and bring the soup to a boil.
Add the barley, and let the soup return to a boil. Reduce the heat to low, cover the soup, and let it simmer for 45-60 minutes or until the barley is tender.
Taste and adjust any seasonings.