With the watch open you will be able to see the two ribbon cables connecting the top and bottom of the watch (red square).

Using a flat tool you can gently separate the cables from the top of the watch.

Parts on the bottom half of the watch that I could identify.

Battery: Held in place by double sided tape. (Center of the watch, blue square).

Speaker: Additional details in Step 3. (Right edge, orange square).

Step/Pedometer Sensor: Connected through the black ribbon cable. (Right side of the bottom edge, yellow square.)

Unknown Sensor: This might be a small proximity or pressure sensor, but I'm unsure. It is connected to the black ribbon cable. (Left side of the bottom edge, pink square.)