Shokz OpenRun 2 is the best headphone for trail running and ultra endurance sports because it keeps your ears open. You hear the trail, the cars, the other runners. In mountains, open ears are safety — not a preference. I've gone through 3–4 pairs over six years. I would not spend a single day without one.
Plus: two hacks below that make the device dramatically more useful — neither is in any Shokz documentation.