Just got my 5X. On first impression, it's not any worse than a Nexus 5, and perhaps slightly better.
The big issue is that wired headsets (Xiaomi Piston) that worked perfectly on the Nexus 5 don't work at all on the 5X. No sound, no mic, and controls don't work. There are threads on xda and androidcentral about this, so I know I'm not alone. No solutions. First hypothesis is that Google/LG changed the headset standard, but we'll see as the situation develops.
Standard headphones (HiFiMan RE-0), work fine. Plenty of power from the 5X to drive them, even at 64 ohms. That's in a quiet room however, the true test will be if I can get enough volume to still be audible on a noisy subway platform. Nexus 5 really couldn't do it without Viper4Android, we'll see how the 5x does in a couple of days.