Changing the volume and reverb levels of each instrument can be achieved in the audio settings of the score.
Audio settings are accessible through the "Playback options" link in the list besides the play button.
By default, all the volumes are set at the maximum level, while reverb is turned off for each instrument.
Each time a slider is changed in the score settings, it is automatically modified within the score data, and synchronized on every device having access to the score. Therefore, every people visiting this score will get the same audio mix that you set there.
Volumes can also be changed locally (not synchronized) through the local audio settings panel.
Once the volume of an instrument has been changed, the playback will be changed but it won't be saved online until the Save button is clicked.
The Restore button will just restore the saved volumes from the score settings.