Mono audio devices, such as Bluetooth HSP/HDP headsets, don't have a suitable icon. When using the sound panel in Control Center to test the audio on these types of device the broken-icon icon is shown. The missing icons are "audio-speaker-mono.svg" and ""audio-speaker-mono-testing.svg".
Actually, I think this might be a bug in gnome-control-center, that seems to be where the icons live.
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