How do I get the software to stop speaking every single letter as it is inserted into the message window?

There are three things that could be causing this to happen:
 
First -

1.  Select the Settings button in the toolbar.


Note: If the Toolbar is hidden, first select the Modify Button  at the top right corner of your screen, then Settings.

2.  Select Interface

3.  Select the Message Windowtab.



4.  Clear the box for Speak on Entry (if it is checked).



Second (if first does not resolve the problem) -

1. Go back to Settings.

2.  Select Access Method.

3.  Select Advanced Settings.



4.  Select the Audio Cue tab.



5. Disable the Audio Cue.
 

Third - Modify the keyboard buttons to remove the keyFeedback() action.

1.  Navigate to the keyboard page.
2.  Select the Modify button in the top right corner of your screen. 

Note:  If the Toolbar is disabled, when you select the Modify button, a dialog will open with two options: Modify or Settings. Select Modify.

3.  Select the first keyboard letter.

4.  On the Properties panel that opens, select the box next to Actions to go into the Script Editor.
 
5.  If there is an action on the button, keyFeedback(), select it.



6.  Select the trash can icon     to delete the action. 

7. Select the green Done button to close the Script Editor.

8.  Continue selecting each keyboard letter, and follow steps 4 through 7 above to delete the action from each letter.