I'll have to think about it. It might confuse users if you can't click a tile you are usually perfectly allowed to click. On the other hand, it should be a very rare case. I can't actually remember having consciously seen any. Another possibility would be to automatically disable the current selection when clicking on a tile you can't swap with, but that could also lower the "fault tolerance" in cases you, for example, click a little too far away. Auto-deselection would force you to click another two times to actually do the move you intended, not just once.
Glad the audio is working, though!