LG G3 external speaker audio cuts out soon after boot

I'm having an elusive problem with my LG G3:

Soon after booting up, the external speaker's audio will cut out and pretty much stop completely. I say "pretty much" because it will occasionally and seemingly randomly start again for a few seconds here and there, then crackle and stop.

Symptoms:

-Only external speaker is affected, not microphone or phone speaker

-Boots up with sound, then cuts out after a short time

-Possibly triggered by extended audio clips?

-Tapping on the phone has occasionally started / stopped the sound. This points to a hardware issue

-Problem seems replicable, but I am not missing the described "metallic-looking fabric" from the inside of my phone:

https://android.stackexchange.com/questi...

Additional symptoms:

-My rear-facing camera also no longer works. Could be unrelated, but maybe not

-After the factory reset, I get errors from "Sprint ID" failing to download every boot. Cool stuff

Things I've tried:

-Clearing cache and data from various related apps

-Factory reset

-Disabled USB routing from developer options

-Disabled power management in battery settings

-Disabled Google Voice

-Booted into Safe Mode (same symptoms)

-Tested via Sprint secret code (external speaker sometimes works, sometimes doesn't; rear-facing camera never does)

-Tested via Soundabout (doesn't work)

-Removed and reinstalled both external speaker and rear-facing camera

- Bending small copper speaker "springs" up towards the facing contacts

-Ordered new external speaker and rear-facing camera and installed those (twice, in the case of the camera)

This is a frustration, obviously... If anyone has any new ideas to fix it, or can at least tell me that my mobo is bunked so that I may move on, please respond!