The HD5770 GPU DOES have the ability to have audio out it's Mini Display Port connections. I have other GPUs also and also was confused, for example, when the RX580 shows an "audio controller" in the PCI info in system profile. However, the HD5770 will not show this....or at least it doesn't on my system. I also already had the "HDMIAudio" kext hack installed when installing that GPU so I am not 100% sure if audio out the mDP connections are supported natively or worked because of the addition of the kext hack. But regardless, it is possible. Again, just not sure if you need the hack or not