Initial music playing code.

mapgen.lua

@@ -120,4 +120,3 @@ function generate_map(start)
       end
    end
 end
-

sound.lua

@@ -3,6 +3,13 @@
 function init_sound()
    -- each name maps to a pattern number to play
    music_map = {
-      desert = 0,
+      desert = 0
    }
 end
+
+-- call this once to initialize biome sound.
+-- by convention channel 4 should always be the
+-- preferred sfx channel, so we unmask it.
+function play_biome_music(biome)
+   music(music_map[biome], 2000, 7)
+end