I've seen some similar issues reported, but they were for specific cars/stereos, and the symptoms were different, so I'm posting about mine:
Phone: Samsung Galaxy SIII, Android 4.3.
Car Stereo: Pioneer AVH-X2500BT
This is a stereo I bought and installed myself. All of its features worked fine, and I was enjoying making calls using voice dialing via the stereo. Then, I installed the 4.3 update to the Galaxy SIII. Ever since (literally, I made a call that day before installing it, and tried shortly after), I cannot use the voice dialing feature. Any attempt to use voice dialing or voice commands via the car stereo will fail. Here's what happens:
I do a long press on the car stereo's Home button to initiate voice dialing. On my phone, S-Voice starts up, and a Toast (little message) pops up briefly that says "Check in-car audio". When I talk, the little animation on the microphone button in S-Voice does not happen, meaning it's not hearing anything. The car stereo shortly cancels out of voice dialing mode, as though it timed out, or something.
I've tried switching to Google voice, but the same thing happens. I've tried all kinds of configuration on both devices. I've unpaired and re-paired. Nothing.
Also, turn-by turn directions from Google Maps appears to not work. The app thinks it's giving them, and when I turn the car off, the voice suddenly works from the phone, but it just won't come out the stereo.
All other features work fine, including Pandora, and streaming Bluetooth audio. I can even make phone calls, by accessing the contacts and recent call list from the car stereo. The calls work and come through the stereo fine. Literally voice dialing and Google Maps are all that doesn't work.
Does anyone know how to fix this?