In upgrading to Ubuntu Trusty recently, I found the volume of Radio Tray to be much higher than the default system sounds. When I turned the system volume down to a reasonable level for Radio Tray, system sounds were inaudible.
I tried changing the volume for Radio Tray in the applications window of Sounds: System Settings -> Sounds -> Applications
For some reason, this didn’t work. On reboot the volume was again at 100%.
I fixed this problem by changing the default volume in the radio tray config file:
I changed line 3 from:
<option name="volume_level" value="1"/> to <option name="volume_level" value="0.05"/>
The value is not linear but logarithmic.
In other words, volume value of .5 will not move the pointer on the volume control to the center. .05 put the control where you see it in image above. You’ll have to change the value and logout/login to hear and adjust the changes to suit your situation.