basically you will need to access your system files. iFile, in Cydia, does have a trial period or there are other methods that can be done on your computer, depending on if you have a Mac or PC
here is that path you are looking for: System > Library > Audio > UISounds > SentMessage.caf
sometimes with certain themes/tweaks it will change or remove this file. if it has been changed more than likely it was changed to SentMessage.xxx it simply needs to be changed back to SentMessage.caf
if it has been completely removed there are a few ways to put it back onto the device
give the first method a try and see what happens. i'll keep an eye on this thread if you need more help and as always, be careful when accessing system files