Notification sound added. Docs updated.

internal/sound/theme_test.go

@@ -32,7 +32,7 @@ import (
 func TestBundledNormalizedPackDecodesAndContainsEveryCue(t *testing.T) {
 	root := "../../config/sound/default"
 	clips, err := load_theme([]string{root})
-	if err != nil || len(clips) != 19 {
+	if err != nil || len(clips) != 20 {
 		t.Fatal(len(clips), err)
 	}
 	paths, err := filepath.Glob(filepath.Join(root, "*.wav"))