I don't think the dev console allows playback. It's more that for some reasons, the page was activated. If you try this, I believe that playback will not work:

1. open new tab (blank)
2. open dev console
3. paste http://googlechromelabs.github.io/web-audio-samples/stress-test/boxes in URL bar

Expected result: no sound
Actual result: no sound

Yes, if you load the page after opening the dev console, it doesn't play.  But if you reload it does as I show in my repro steps.

On the other hand, http://rtoy.github.io/webaudio-hacks/tests/osc.html starts playing right away without interaction from me. (ToT today).  But other examples from http://rtoy.github.io/webaudio-hacks/tests/index.html don't play audio, as expected, even with user interaction. (Because I haven't updated the examples to support user gestures to play.)

mlamouri: Any thoughts on my osc.html test in c#2?  Autoplay should be consistent and refuse to produce audio here.

I investigated this a little bit and it has nothing to do with Web Audio. What happens is that when the console is opened (via a shortcut), it sets the user gesture flag on the tab. When reloading from the UI, the flag is then set as a navigation user gesture instead of not being set (behaviour without the console open).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/decbccdbfc17bd75df40a9c47b24dd80dfea90c0

commit decbccdbfc17bd75df40a9c47b24dd80dfea90c0
Author: Andrey Lushnikov <lushnikov@chromium.org>
Date: Mon Jul 23 23:40:49 2018

DevTools: pass parsed message as unique_ptr instead of raw pointer

This will allow devtools_session to own parsed message and
bind it as a part of a callback.

R=dgozman

Bug:  817413 
Change-Id: I48dc7a12741902603e22233cdd26b6bb97ecf453
Reviewed-on: https://chromium-review.googlesource.com/1145954
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577328}
[modify] https://crrev.com/decbccdbfc17bd75df40a9c47b24dd80dfea90c0/content/browser/devtools/browser_devtools_agent_host.cc
[modify] https://crrev.com/decbccdbfc17bd75df40a9c47b24dd80dfea90c0/content/browser/devtools/browser_devtools_agent_host.h
[modify] https://crrev.com/decbccdbfc17bd75df40a9c47b24dd80dfea90c0/content/browser/devtools/devtools_agent_host_impl.cc
[modify] https://crrev.com/decbccdbfc17bd75df40a9c47b24dd80dfea90c0/content/browser/devtools/devtools_agent_host_impl.h
[modify] https://crrev.com/decbccdbfc17bd75df40a9c47b24dd80dfea90c0/content/browser/devtools/devtools_session.cc
[modify] https://crrev.com/decbccdbfc17bd75df40a9c47b24dd80dfea90c0/content/browser/devtools/devtools_session.h
[modify] https://crrev.com/decbccdbfc17bd75df40a9c47b24dd80dfea90c0/content/browser/devtools/target_registry.cc
[modify] https://crrev.com/decbccdbfc17bd75df40a9c47b24dd80dfea90c0/content/browser/devtools/target_registry.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bd8a9141c0588fd211570c769d82dfa595add19a

commit bd8a9141c0588fd211570c769d82dfa595add19a
Author: Andrey Lushnikov <lushnikov@chromium.org>
Date: Thu Jul 26 01:59:04 2018

DevTools: trigger browser-side reload when devtools is open

DevTools should initiate browser-side reload so that reload button doesn't
change behavior with and without devtools open.

R=dgozman

Bug:  817413 
Change-Id: Ia69070c82e7ea46e165c485081ab2fe965fae531
Reviewed-on: https://chromium-review.googlesource.com/1107118
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578167}
[modify] https://crrev.com/bd8a9141c0588fd211570c769d82dfa595add19a/content/browser/devtools/protocol/page_handler.cc
[modify] https://crrev.com/bd8a9141c0588fd211570c769d82dfa595add19a/content/browser/devtools/protocol/page_handler.h
[modify] https://crrev.com/bd8a9141c0588fd211570c769d82dfa595add19a/content/browser/devtools/protocol_config.json
[modify] https://crrev.com/bd8a9141c0588fd211570c769d82dfa595add19a/third_party/blink/renderer/core/inspector/inspector_page_agent.cc

I tried today on trunk (70.0.3508.0) and it's working. Thanks!