SUMMARY
When plugging my headphones into the front 3.5 mm jack of my desktop pc the output of my speakers that are connected via IEC958 (aka S/PDIF) is not muted but actually increased.

STEPS TO REPRODUCE
1. Have music playing over IEC958. Volume can be adjusted. The only active pulseaudio sink is alsa_output.pci-0000_00_1b.0.iec958-stereo
2. Plug in headphone in front 3.5 mm jack.
3. Music is playing through headphones and this volume can be adjusted. The only pulseaudio sind is alsa_output.pci-0000_00_1b.0.analog-stereo. BUT speakers are still playing, the volume increased immensly and volume cannot be adjusted by any known means.

OBSERVED RESULT
Sound is still playing over speakers

EXPECTED RESULT
Sound is only playing through headphones and speakers are muted.


SOFTWARE/OS VERSIONS
Operating System: KDE neon 5.16
KDE Plasma Version: 5.16.4
KDE Frameworks Version: 5.60.0
Qt Version: 5.12.3
Kernel Version: 4.18.0-25-generic
OS Type: 64-bit
Processors: 4 × Intel® Core™ i5-3570K CPU @ 3.40GHz
Memory: 15,6 GiB of RAM


ADDITIONAL INFORMATION
Tested with mpv and Firefox.

SINK LIST WITHOUT HEADPHONES
$ pactl list sinks
Sink #74
        State: RUNNING
        Name: alsa_output.pci-0000_00_1b.0.iec958-stereo
        Description: Built-in Audio Digital Stereo (IEC958)
        Driver: module-alsa-card.c
        Sample Specification: s16le 2ch 44100Hz
        Channel Map: front-left,front-right
        Owner Module: 8
        Mute: no
        Volume: front-left: 39322 /  60% / -13.31 dB,   front-right: 39322 /  60% / -13.31 dB
                balance 0.00
        Base Volume: 65536 / 100% / 0.00 dB
        Monitor Source: alsa_output.pci-0000_00_1b.0.iec958-stereo.monitor
        Latency: 16922 usec, configured 20834 usec
        Flags: HARDWARE HW_MUTE_CTRL DECIBEL_VOLUME LATENCY SET_FORMATS 
        Properties:
                alsa.resolution_bits = "16"
                device.api = "alsa"
                device.class = "sound"
                alsa.class = "generic"
                alsa.subclass = "generic-mix"
                alsa.name = "ALC898 Digital"
                alsa.id = "ALC898 Digital"
                alsa.subdevice = "0"
                alsa.subdevice_name = "subdevice #0"
                alsa.device = "1"
                alsa.card = "0"
                alsa.card_name = "HDA Intel PCH"
                alsa.long_card_name = "HDA Intel PCH at 0xf7b10000 irq 35"
                alsa.driver_name = "snd_hda_intel"
                device.bus_path = "pci-0000:00:1b.0"
                sysfs.path = "/devices/pci0000:00/0000:00:1b.0/sound/card0"
                device.bus = "pci"
                device.vendor.id = "8086"
                device.vendor.name = "Intel Corporation"
                device.product.id = "1e20"
                device.product.name = "7 Series/C216 Chipset Family High Definition Audio Controller (Z77 Extreme4 motherboard)"
                device.form_factor = "internal"
                device.string = "iec958:0"
                device.buffering.buffer_size = "65536"
                device.buffering.fragment_size = "32768"
                device.access_mode = "mmap+timer"
                device.profile.name = "iec958-stereo"
                device.profile.description = "Digital Stereo (IEC958)"
                device.description = "Built-in Audio Digital Stereo (IEC958)"
                alsa.mixer_name = "Realtek ALC898"
                alsa.components = "HDA:10ec0899,18491898,00100003 HDA:80862806,80860101,00100000"
                module-udev-detect.discovered = "1"
                device.icon_name = "audio-card-pci"
        Ports:
                iec958-stereo-output: Digital Output (S/PDIF) (priority: 0)
        Active Port: iec958-stereo-output
        Formats:
                pcm


SINK LIST WITH HEADPHONES
$ pactl list sinks
Sink #76
        State: IDLE
        Name: alsa_output.pci-0000_00_1b.0.analog-stereo
        Description: Built-in Audio Analog Stereo
        Driver: module-alsa-card.c
        Sample Specification: s16le 2ch 48000Hz
        Channel Map: front-left,front-right
        Owner Module: 8
        Mute: no
        Volume: front-left: 4588 /   7% / -69.29 dB,   front-right: 4588 /   7% / -69.29 dB
                balance 0.00
        Base Volume: 65536 / 100% / 0.00 dB
        Monitor Source: alsa_output.pci-0000_00_1b.0.analog-stereo.monitor
        Latency: 20287 usec, configured 20834 usec
        Flags: HARDWARE HW_MUTE_CTRL HW_VOLUME_CTRL DECIBEL_VOLUME LATENCY 
        Properties:
                alsa.resolution_bits = "16"
                device.api = "alsa"
                device.class = "sound"
                alsa.class = "generic"
                alsa.subclass = "generic-mix"
                alsa.name = "ALC898 Analog"
                alsa.id = "ALC898 Analog"
                alsa.subdevice = "0"
                alsa.subdevice_name = "subdevice #0"
                alsa.device = "0"
                alsa.card = "0"
                alsa.card_name = "HDA Intel PCH"
                alsa.long_card_name = "HDA Intel PCH at 0xf7b10000 irq 35"
                alsa.driver_name = "snd_hda_intel"
                device.bus_path = "pci-0000:00:1b.0"
                sysfs.path = "/devices/pci0000:00/0000:00:1b.0/sound/card0"
                device.bus = "pci"
                device.vendor.id = "8086"
                device.vendor.name = "Intel Corporation"
                device.product.id = "1e20"
                device.product.name = "7 Series/C216 Chipset Family High Definition Audio Controller (Z77 Extreme4 motherboard)"
                device.form_factor = "internal"
                device.string = "front:0"
                device.buffering.buffer_size = "65536"
                device.buffering.fragment_size = "32768"
                device.access_mode = "mmap+timer"
                device.profile.name = "analog-stereo"
                device.profile.description = "Analog Stereo"
                device.description = "Built-in Audio Analog Stereo"
                alsa.mixer_name = "Realtek ALC898"
                alsa.components = "HDA:10ec0899,18491898,00100003 HDA:80862806,80860101,00100000"
                module-udev-detect.discovered = "1"
                device.icon_name = "audio-card-pci"
        Ports:
                analog-output-lineout: Line Out (priority: 9900, not available)
                analog-output-headphones: Headphones (priority: 9000, available)
        Active Port: analog-output-headphones
        Formats:
                pcm