There are a couple of threads on disabling the "Message Sent" notification bubble, such as this thread. Here's something different. If you still want the message sent notification to appear but in a different way, you can also do so.

Registry Key: HKCU\Control Panel\Notifications\Default
Registry Value: Options
Registry Data: 0 to 11

Here are the description for the different registry data values:
0 - show icon
1 - show icon and play notification sound
2 - show icon and vibration
3 - show icon and vibration and play notification sound
4 - same as 0
5 - same as 1
6 - same as 3
7 - same as 3
8 - show icon and notification bubble
9 - show icon and play notification sound and show notification bubble
10 - show icon and notification bubble and vibration
11 - show icon and play notification sound and show notification bubble and vibration

Source

Finally a solution for those who dislike the 'beep'

Thanks bro.

This really helps. The beep has really got on my nerves.

Just wondering,
mine shows a value at 13 now..
that's y i'm hesitant to change it to '1'.

is 13 the default value?
It's vibrating, sounded and notification boxed now.

thanks =)

13 yields the same result as 1

how come it's irritating to you guys?

i think it pretty much indicates n let me know my msg has been successfully sent out

Thanks for that

Think it would be better if it beeps or notify ONLY when it is UNSUCCESSFUL in sending, is there a way to do that?