It seems that Google enables the Qualcomm’s aptX Bluetooth streaming on Nexus 5X in Android O. Developer Preview 4 shows this option, but I am not cent percent sure about it. Note that Google already confirmed that aptX and aptX HD Bluetooth audio streaming won’t work on Nexus 5X and 6P. Nonetheless, you can see the aptX option under Bluetooth audio codec menu in Developer options.
Bluetooth aptX is an audio streaming protocol by Qualcomm and it will be a new feature of Android O (Android 8.0). Qualcomm aptX audio technology enhances the wireless sound quality, so you can experience crystal clear sound on Bluetooth enabled devices. Previously, Bluetooth enabled devices are known for providing average quality sound via Bluetooth.
Comes to the Nexus 5X and Nexus 6P, both phones have the Qualcomm WCD9330 audio codec chip, which does support aptX. So chances for getting Qualcomm’s aptX Bluetooth streaming on these devices is far above the ground.
However, it is unclear why Google excluded Nexus 5X and Nexus 6P from getting aptX support. It may be due to Qualcomm’s aptX requires some other software or hardware components to work on these devices.
If you are interested and want to know more about Qualcomm’s aptX Bluetooth streaming technology, you can read more about it at Qualcomm’s website here.