[smotrs]

This is the method I used, hope it's useful.

[\HKCU\ControlPanel\Notifications\Default]
"Options" = dword:X

X being one of the values listed below (Default on Sprint was 13). Personally, I used option 1, I like the audible but couldn't stand the bubble.
Changes take effect immediately

0 - Icon
1 - Icon and Audio
2 - Icon and Vibrate (SSS)
3 - Icon, Audio and Vibrate (L)

4 - Same as 0
5 - Same as 1
6 - Same as 2
7 - Same as 3

8 - Icon and Bubble
9 - Icon, Bubble and Audio
10 - Icon, Bubble and Vibrate (LLS)
11 - Icon, Bubble, Audio and Vibrate (L)

12 - Bubble
13 - Bubble and Audio
14 - Bubble and Vibrate (SSL)
15 - Bubble, Audio and Vibrate (L)

The following do not reset the keyboard after sending message. What that means is that as you type and the software is doing the word complete, after sending, the word complete dialog still shows on the screen for the last word typed.

16 - Icon and No KB Reset
17 - Icon, Audio and No KB Reset
18 - Icon, Vibrate (SSS) and No KB Reset
19 - Icon, Audio, Vibrate (L) and No KB Reset

20 - Same as 16

I stopped trying values when I got to 20 since from 16 on they appeared to be the same as 0-15 but with the KB issue I mention above. Also, all the S's and L's represent quantity and length of the vibrations. I tried each setting about 10 times and 3 of the 10, the last vib was opposite of what I show. In most cases though, it did as I show above.

After I found these, I did a search and found that a few others had found and posted the above as well. So I wasn't the first, but I did find it on my own.