It's actually 12 seconds. You can change this value by adding/modifying the following 
registry:
Key:
HKEY_CURRENT_USER\Software\Patchou\MsgPlus2\your@email.com\Preferences -> 
ContactSoundMaxTime (DWORD)
Description:
When you associate a sound to contact sign-in or sign-out, it is played 12 seconds by default. You can use this parameter to decrease or increase this time between 1 and 100 seconds.
In case you don't know how to edit your registry: Click 
Start, select 
Run, type 
regedit in "Run" window, and click OK 
