The 6p is known to have weak output from its headphone jack. It's good quality audio, but it's just not loud enough. I got around the problem by rooting my phone and using a custom kernel to boost the output and now it's more than loud enough!
This is the best part about owning a Nexus. If you don't like something about your phone, 90% of the time you can fix it with root and a kernel, ROM, or an xposed module.
Posted via my Nexus 6P on VerLIEzon Wireless but without their shackles.