Within EnGage, choose the site and click the assignments tab.  There, edit volume within Site Settings.  Adjust both the Primary and Secondary volumes seen below.  

Note: Primary volume and Secondary volume should be the same number.

Save the changes and republish site settings.

Note: If certain files within the program seem to be louder than the rest, you can edit the volume on a content level.  Identify the file and click edit. Within the General tab, edit the primary volume.  Click OK and republish the piece of content.  

Note: You do not need to republish the timetable where the content is playing.  The volume adjustment will apply to the site/s after you republish the piece of content.  Expect the content's volume to adjust the next time it plays within the program.