Okular's frameworks branch (or maybe it's been merged to master by now, not sure) is optionally using QtSpeech.
KNotifyConfig and KNotification are optionally using QtSpeech as of the last frameworks release already.
KHangMan and KAnagram have been using QtSpeech optionally since December of last year or so.
This leaves the big one, KMouth. Unfortunately KMouth has been bitrotting since about 2006 or so. All commits since have been minor or bug fixes. Many because KFoo classes changed and were used in KMouth. It's master branch still uses Qt3Support and K3ListItem, etc.
I started a couple of months ago to start porting it away from Qt3Support and K3* so it can be ported to Qt5 and QtSpeech, but it's been a long slow process. Current progress can be seen on the noqt3support branch, but even the last commit there is from a couple of months ago. Part of the trouble has been getting the PhraseBookDialog with it's accompanying model and treeview to work as it used to including drag and drop, copy/paste, import, export, save/load, etc.. Many bug fixes are also in the works, and I am positive it will be better than before once it's done, but it's taking time since I don't want to break loading phrasebook files of any existing users (If there are any out there, please shout, I'd love to hear from you about how you use KMouth).
Once KMouth is ported to QtSpeech I believe most/all users of the old KSpeech dbus api will be safely using the new QtSpeech library.
P.S. Once it's ported to Qt5/KF5 and QtSpeech KMouth could use some updating. A couple of years ago I saw a fancy speech application on the evening news that enabled a young man to speak with his family by tapping icons on an ipad which then spoke for him. KMouth could be useful in the same way with a bit of polish in my opinion.