I had the same problem with Galaxy S5 -- Android 4.4.4 (Kit Kat). After much Googling (found nothing) and discovered this cure for me. I use "DASHLANE" and had tried using the special DASHLANE KEYBOARD which I think, in hindsight, caused my problem. So if you do not use the Dashlane Keyboard this may not work for you. If you have activated the Dashlane Keyboard, go to Settings, Accessibility (under System), scroll to the bottom to Dashlane and open. Turn the green toggle in the upper right-hand corner to OFF. Then go back to S Voice and the "Enable Voice Feedback" is now not grayed out (at least for me). I enabled voice feedback, and it now worked. I then went back to Accessibility/Dashlane and turned Dashlane back on and S Voice's voice feedback still worked.
Hope that helps. If you don't have the Dashlane Keyboard I don't know what else to tell you. Just try toggling other audio settings and going back to S Voice to see if it had any effect.
BTW, Dashlane is a great password management tool and while the free version is good, in my opinion, it is well worth it to pay for the Premier upgrade to be able to sync and access your passwords between your desktop/laptop and all of your mobile devices. So, since it is a great product otherwise, I am not upset with Dashlane because of this hiccup.
Addendum 1: After posting the message above, I discovered I had to leave the Dashlane keyboard off all of the time for the S Voice to work.
Addendum #2: FYI, Dashlane wrote concerning this issue:
Yes, there is a small bug with this and here’s how to fix it:
For Samsung devices:
• Go to Settings > Accessibility > Vision > Talkback “ON”
• Select “Settings” from the bottom of the page
• Uncheck all Talkback options
• Switch Talkback “Off” again
• Go back to Settings > Application manager > All
• Disable Google TTS (text to speech)
• Disable Samsung TTS (text to speech)
If you’re experiencing the same issue on another phone, try this:
• Go to Settings > App Manager > Select GoogleTTS > Disable
• Go to Settings > App Manager > Select SamsungTTS (on a Samsung phone) > Disable