The presence of gum or sap on cherry trees is a natural response to expel insect pests and diseases or a response to some type of environmental stress. Boring insects, which are the larval stage of moths, are common on cherries, as are canker diseases. There is unfortunately not much to be done for them. https://extension.umd.edu/hgic/topics/borers-trees
Mulch placed right up against the trunk of the tree can make an inviting habitat for borers. It also looks like the tree was planted too deeply, which is a stressor that can lead to decline of trees in general. On a properly planted tree, you should be able to see the top of the root flare right at the soil line.

At this point, unfortunately, your tree will simply limp along. We recommend that you plan for removal and replacement. In general cherry trees are not long-lived. You may want to take the opportunity to select another species of tree for your site.
If you want to keep it and let the tree run its course, prune out any dead branches that occur, pull back the mulch about 6" from the base of the tree, and water during dry periods.  

Christa