The default volume value is still 1.0, but the value now used is whatever you set on the config page or 1.0 if you never touch the setting. if you take slider all the way to the right, it will be the default of 1.0
Like I did mention, your's seemed an odd request because I have never heard anyone mention that the text to speech was too loud. I expect there was some odd situation on your device where either the text to speech engine was disproportionately loud or the song playback was disproportionately quiet and now it has fixed itself.
I guess the slider setting will help if anyone else's device gets in that funky situation.