Certified GKH Gardening Expert
Unfortunately, without photos I can't say much about the situation. Fungi are tricky, and you can worsen an issue by trying to treat it, but it can be an even bigger issue if it happens to be a detrimental fungus.
Generally, though, plants and fungi thrive in totally different conditions. If it is ideal for one, it is not ideal for the other. In most cases, the mushrooms that survive in less than ideal conditions are not detrimental to plants or trees.
Ir order to rid the area of mushrooms, you must remove the conditions in which they thrive. Most of the times, this will simply mean that the soil remains way too wet. Watering once the soil (NOT THE MULCH) dries down to about 3 or 4 inches, thoroughly. Though, in your area, there is rarely enough of a drought to water at all with mulch.
Treating with a fungicide may or may not help. Mushrooms generally won't respond to them quickly. This will need to be done over a whole season, along with keeping the area dry.
Here are some articles that will help you: