What's an example of using audio to trick the user?  I'm always excited to hear more innovative ways of tricking users...

We can probably do this, it just seems a little surprising to me.

I'm a little concerned that muting the audio is excessive.  In my mind, the audio is part of the page content, which we (theoretically) want to provide.  Just as we don't black out the DOM, it seems like we should be turning off the audio.

It seems like this is something that would be better handled by either
a) inline install hardening, which prevents any content (audio or visual) from the page from interfering or
b) having better browser-wide indications for when audio might be undesirable

Muting it *only* for extension inline installation seems a little arbitrary, given it is absolutely annoying, but not (so far as I can see) a real security concern (unlike e.g. fullscreen, which hides the url, iframes, which make it difficult to see where the item is coming from, etc).

I'm open to being convinced otherwise, but I'm worried that this will hurt potential good use cases (and I don't think that audio is anywhere near as problematic as the DOM itself, with arrows pointing up to "click this to get your free pony!").

On my platform (Linux, Chrome 55) Chrome already pauses playback of audio tags during the dialog triggered by an onbeforeunload handler. On closer look, I think this is a side effect of something else (as the audio continues playing for ~1 second and this doesn't seem to apply to videos). I believe this change (muting audio) should also be applied to the onbeforeunload dialog, so I agree that this change could and should be part of a wider effort.

In the case that inspired the request, the audio was a loop of a firm voice repeating "click add button to install extension" while the site used a combination of other tricks to make it hard for the user to leave and told the user that they should install the extension to be allowed to leave.

By itself, the audio wouldn't be that much of a problem, but I suspect it is very effective at further raising the stress level of the user, making it more likely that they will make a bad decision (installing the malicious extension). Even knowing how everything works and that I'm not really at risk, the experience was surprisingly intimidating.

The idea is that the extension dialog should try to create a "safe" environment for the user to make an informed decision. Indeed, blacking out or darkening the DOM crossed my mind too. Even in cases where legitimate content is being played, interrupting any music etc. will make the user pay attention to the dialog. Given that the dialog is asking the user to make an important trust decision, I don't think that's a bad thing.

I don't think there are any critical use cases that will break by muting audio during an extension install dialog. Compare with the decision to remove the ability to provide a custom onbeforeunload dialog text, which impacted legitimate use cases but was still made to prevent abuse:
https://groups.google.com/a/chromium.org/forum/#!msg/blink-dev/YIH8CoYVGSg/Di7TsljXDQAJ

Inline install is deprecated:

https://blog.chromium.org/2018/06/improving-extension-transparency-for.html