I'm splitting this one away from  bug 637568 . 
I think the problems we're seeing with this related to that the machine was replaced and got a new sound card installed (bug 625808 covers all of that).

Like ivoc@ commented in  bug 411297 , the regressions happened in three steps:
1. Large drop from 3.5 to 2.3 at 2016-08-11 (around #411297). This matches with Edward's first re-enabling of the tests in https://codereview.chromium.org/2233943003

2. Small drop to 2.1 at 2016-08-12 (around #411576). The above enabling was reverted in https://codereview.chromium.org/2237313003 at this day.

3. Small drop to 1.9 at 2016-08-29 (around #415034). Don't know what caused this, Edwards final reland in https://codereview.chromium.org/2241223002 happened at Aug 17th with no noticeable change.

Edward: do you think manual configuration of the output level on the machine could be related to the different values here? Maybe worth taking a look at the machine once and then monitor the next build?