The documentation for the audioNotification capability provides for an optional volumelevel argument on all the available commands. The volumelevel argument is properly exposed in the device presentation on most of the commands, but not on playTrack or playText.
Yet, curiously, the option appears to be available to webCoRE:
I'm currently working on a driver and am seeing issues related to the inconsistency. I expect I can work around it, but am curious why it was done this way.