There are several ways to edit the volumes and dynamic range in Sinfonia, so you may choose which one best suits your case:

If you feel that the overall show is just too dynamic, you may consider modifying the dynamic range preference.

Please see the FAQ about limiting/expanding the Dynamic Range of your show:

If you like the dynamic range, but find that a couple songs stand out, you can edit the volume of an entire song by going into edit mode and moving the output mixer window’s slider for that song. Moving the output mixer volume slider in edit mode will change the overall volume for the currently selected song, which can then be saved.

Here is the section of the manual that deals with the output mixer window.

For fine tuning the volumes, you may want to do so at the instrument level - you can edit volumes for individual, or groups of instruments. This can also be done for a whole song, or a selection of measures.

Here is the section of the manual that deals with the instrument mixer window.