I'm using the EVGA GeForce GTX 750 ACX FTW with the driver version 361.28. The version of the OS is Elementary OS 0.3.2 Freya 64 bit. All the updates and upgrades are installed. I plugged an HDMI cable in it and the other end to a HDMI to otpical cable converter for the sound because my surround sound speakers (Logitech Z906) set only accept optical cable. I've set the audio in System settings to HDMI and the profile to Digital Surround 5.1 (HDMI) Output. I made sure that the video files I'm watching have an audio track that has a 5.1 sound. I tried to watch them with VLC 2.1.6 and Elementary OS own Video player. I used MakeMKV to rip my Blu-Rays. But still, I only get stereo sound no matter what.
this work for me,
you have to install
sudo apt-get install pavucontrol
and select your card sound