Thank you to EpaddYC for the tip-off for this one. There are new volume options available on Stadia for voice chat as you have been given 3 options in your party chat. You can change the Master Volume, Game volume, and Voice Chat volume. At present, it appears that is only available if you are playing on PC. I have checked on Chromecast Ultra and you can only change the voice chat option.
These options will allow you to adjust more settings to get the balance right.