I'm using Lubuntu 17.10 fully updated almost clean install on a Macbook Pro Retina late 2013. Audio is working perfectly fine through both inbuilt speakers and headphones. My headphone jack is sending a red light out constantly. I've tried disabling IEC958 through amixer and alsamixer. This does not disable the red light and once pulseaudio gets restarted it reverts to default (on). (It doesn't happen and never has on OSX).
$ amixer
Simple mixer control 'IEC958',0
Capabilities: pswitch pswitch-joined
Playback channels: Mono
Mono: Playback [off]
Simple mixer control 'IEC958',1
Capabilities: pswitch pswitch-joined
Playback channels: Mono
Mono: Playback [off]
Simple mixer control 'IEC958',2
Capabilities: pswitch pswitch-joined
Playback channels: Mono
Mono: Playback [off]
Simple mixer control 'IEC958',3
Capabilities: pswitch pswitch-joined
Playback channels: Mono
Mono: Playback [off]
Simple mixer control 'IEC958',4
Capabilities: pswitch pswitch-joined
Playback channels: Mono
Mono: Playback [off]
I also tried ylwghst's solution found at Help me kill this red light coming from headphone jack? . This didn't solve the problem (might have something to do with my limited understanding of the config file - it did make my computer think the microphone was a speaker, and did disable all audio-capabilities).
If I go to pulsaudio and change config to analog surround it does fix the red light, however it also disable all sound-capabilities.
I also tried wutangforever's suggestion found on ALSA Mixer settings won't save (Kubuntu) . This did disable the red light without interfering with sound settings, however once I start playing any sound the red light comes back. Once the sound is disabled for a couple of seconds it turns off. This is better, but would still prefer it completely disabled.