I realise I’m 6 months late to the party, but I’m using Google Assistant (which Google Home uses) on my phone. Harmony integration has been added to the Home Control settings (you may need to have your primary language set to US English).
So you can now say “OK Google, ask Harmony to increase the volume” and Harmony will perform the action based on the current activity.
More details are available at myharmony.com/google-assistant
Unfortunately we can’t leverage this in Smartthings because there’s no way (that I know of) to run a Google Assistant action unless you want to go down the “Use a Sonos speaker to activate a voice-controlled device in my home” which is a bit too meta for my liking