Dynamically manage audio routing

Something is needed to manage audio routing according to policies specified both statically and by user configuration. A preliminary set of use cases is as follows:

• Ring tones and notification sounds should play through the speakerphone and not the earpiece.
• Call audio should be switchable between the speakerphone and earpiece.
• Media playback, notification sounds and call audio should be routed to a Bluetooth headset when one is connected.
• Media playback should be muted when a call comes in.
• While using turn-by-turn navigation, the navigation audio should be reduced in volume or muted when a call comes in.
• Audio routing policy should integrate with other feedback settings (#87 (closed))

Related issues:

• #103
• #87 (closed)
• Apps_Issues#60
• calls#43 (closed)
• calls#101 (closed)
• gnome-clocks#18

Prior art:

• Automotive Grade Linux
  • Advanced Audio Architecture: https://docs.automotivelinux.org/docs/en/guppy/apis_services/reference/audio/4a-framework/README.html
  • WirePlumber: https://fosdem.org/2020/schedule/event/ema_pipewire/
• GENIVI
  • GENIVI Audio Manager: https://at.projects.genivi.org/wiki/display/PROJ/Audio+Manager
• Tizen:
  • Murphy: https://01.org/murphy https://github.com/intel/murphy
  • Murphy and GENIVI Audio Manager: https://wiki.tizen.org/IVI/Genivi_Audio_Manager_%26_Murphy_Demo
  • Audio Framework: https://developer.tizen.org/development/guides/native-application/media-and-camera/sound-manager?langswitch=en
• Mer (/Meego/Maemo/Sailfish/Nemo Mobile)
  • Pulseaudio modules: https://github.com/nemomobile?utf8=%E2%9C%93&q=pulseaudio https://git.sailfishos.org/mer-core?utf8=%E2%9C%93&filter=pulseaudio
• ChromeOS
  • Chromium OS Audio Server: https://www.chromium.org/chromium-os/chromiumos-design-docs/cras-chromeos-audio-server

Pulseaudio mailing list feedback: https://lists.freedesktop.org/archives/pulseaudio-discuss/2019-December/031563.html