Step 9: Home stretch . . .
Now for the speaker. The red wire goes next to the switch (picture 2). Polarity does matter. Don't use the battery holder hole in the middle. Solder the leads in place.
The battery holder needs a tweak. One of the legs is in the way of where we want the speaker to be. Trim it off with a knife (picture 3).
Push the speaker through the opening and put the battery holder in place (picture 4). Solder it in place.
The speaker is loud, even using just three volts. I put a small square of double-stick tape over the opening, and then tape the opening to the bottom of the battery holder. That leaves it flush with the bottom of the board (picture 5).
You can use an old toothbrush and alcohol to remove the flux if you are as sloppy as me.
What are the odd that it works? Time to find out . . .