Anyway to disable earphone volume warning?

Are you rooted? Are you on a custom rom? Are you running other modules?
You should be able to download Xposed Framework apk. Install apk through your file manager. Open Xposed, select install/update, let it install then reboot phone. Install the no volume mod apk with file manager, reboot. Then open Xposed app, under the modules tab tick that mod then reboot yet again.

Have you tried Wanam Mod, or Xblast for Xposed Framework? Just install one of those the exact same way as above before you change any settings. After you have installed and rebooted then make the change in Wanam under Sound then reboot.
You must be rooted.
If you're on a custom rom not all Xposed mods play nice with custom roms framework so make sure you have a working nandroid.

(you may have tried all this but I elaborated for all, let us know if you can get it going)

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Thank you so very much.
 
OK, I first saw this issue posted on these forums in 2012, and STILL Google IGNORES their users! And for me this is not because of an old OS or proprietary phone software because this is a Nexus 5! I'm not interested in modding my phone, a factor in why I chose a Nexus. And as for "safety" - it is still confusing when I plug it in the car and expect it to work, whether it is the car stereo source or some problem with the stereo or the fact that Android has locked my volume down again - the safety issue for me is will this cause an accident!? I'm surprised Google isn't facing any lawsuits for something along these lines. I find the prospect of a driverless car produced by this company extremely disturbing. And I find this annoying piece of unusabilty increasingly frustrating and infuriating. Shame on you Google-Android.