some variables not referenced in another translation unit made static

Also adds some cross-refence comments for some variables that are
referenced in another translation unit.

src/music.c

@@ -472,7 +472,7 @@ generic_lvl_desc(void)
         return "dungeon";
 }
 
-const char *beats[] = {
+static const char *beats[] = {
     "stepper", "one drop", "slow two", "triple stroke roll",
     "double shuffle", "half-time shuffle", "second line", "train"
 };