To set the volume for each sound choose Settings > Audio > Choose volumes... or press Ctrl+A. A window will open allowing you to set volumes.
To set a sound's volume, click on the corresponding slider and drag it up or down to the desired level. To immediately apply a volume and play a test, click the corresponding Apply & test button below its slider.
To apply all chosen volumes and close the window, click OK. To discard any changes (that haven't been applied and tested yet) and close the window, click Cancel.