Bulbs are built to take the cold weather, so try not to worry. Some bulbs such as grape hyacinth routinely sprout in the fall; others may show some leaves if the fall is a long warm one. Over winter, although the foliage may become a bit bedraggled, it will not hurt the plants as long as the flower buds are not showing. Adding soil might cause them to rot. You might toss a bit of straw or other light fluffy mulch such as evergreen boughs over the area, but in most cases it isn’t necessary. Of course, you will certainly need to remove any mulch layer in the spring so they can sprout normally as the weather moderates.