Google Chrome 67.0.3363.0 (Official Build) canary (64-bit)
Samus Chromebook

Steps to repro:
Start ChromeVox with ctrl + alt + z

Adjust pitch or speed of Chromevox using these keyboard shortcuts: 
ChromeVox + Shift + Close Bracket = Decrease pitch of speech
ChromeVox + Shift + Open Bracket = Decrease rate of speech
ChromeVox + Close Bracket = Increase pitch of speech
ChromeVox + Open Bracket = Increase rate of speech

Expected: ability to quickly go back to default, especially since both defaults are not set at 50 (default speed seems to be around 21%)
Actual: no keystroke to go to defaults