After T5369, we should have infrastructure for playing sounds without duplicating them across windows.
- Make or find at least one permissively licensed sound which no one hates too much.
- Add a settings panel to let users disable sounds (Settings → Sound, provide a "never play any sounds" option at a minimum).
- Add the sound for chat in an open thread.
Things we might want to pursue eventually, but which are not in scope for this task:
- Special sound for a username mention (see T7580).
- Several sound options.
- Selectable sounds per thread.
- Mute sounds for a given thread.
- Sounds for other notifications?