getDisplayMedia() and system audio

Howdy,

The Screen Capture <https://w3c.github.io/mediacapture-screen-share/> spec
for getDisplayMedia() currently does not handle the case of screen share
with system audio (issue
<https://github.com/w3c/mediacapture-screen-share/issues/61>) but it is
being considered.
- Is this something we are interested in supporting? E.g. {audio:true}.
Please discuss.

Use case: I'm presenting something and I want to show an audio & video clip
as part of the presentation.

With getDisplayMedia(), the user can decide whether to only present a
window or the entire screen. It is not allowed to constrain source
selection, and display capture sources are not enumerable (e.g. open
windows) since that would reveal too much information about the system.
Some questions if we want screen share with audio...

When only presenting a window, should "audio" include only that window's
audio or all system audio?
Should what audio to use be up to the user (similar to choice of display
surface being up to the user) or something that you either accept or
reject? (Application asks for audio and video, I only want to accept video?)

There is a risk that the choices or implementations become complicated. We
should keep things simple, especially since the request means a popup and
needs to be accepted or rejected by the user. If you share a window + audio
and the the remote party hears something different than what you hear due
to sound coming from different windows that might be unexpected and
confusing.

For presentations + conferences, echo of participants talking might be
problematic.

What do people want?