How to lower the volume of Skype sound effects? Skype Version 7.36 (507), OSX Mavericks on 2011 iMac

Skype's sound effects are insanely loud.  They play at the volume of the overall Mac, which is up full and should be.  But everything else on the computer is an appropriate volume (system sounds, browser, etc.), Skype is just way WAY too loud.

This is only its chat and related sound effects, NOT calls.  Has nothing whatsoever to do with voice calls.

Skype does have its own volume control, but it also does absolutely nothing, both for the sound effects AND calls.  Has no effect whatsoever.

Is there any way to quiet just Skype down?

EDIT: Another observation in case it's somehow relevant - changing Skype's audio devices in preferences also does nothing.  All sound effects (non-call related) always come out the internal speaker regardless of what device I select.
What you can do is to disable the sound effects, for each individual "event" (incoming call, new message received, etc).
Or, even better, you can associate to each event a different sound effect, maybe with a different volume.

To do this, you need to go to the preferences, and choose the "Notifications" panel. Then, you choose each individual "Event" for which you want to turn off the sound (via the first drop-down menu), and turn it off by means of the "Play sound" check button.
You can also choose a different sound effect (maybe with a different volume) by choosing a different audio file. This is done using the drop-down menu next to "Play sound", and selecting the last option: "Add/Remove...".
You can then select your file using the "+" button on the bottom-left corner of the small pop-up window.
To have different volumes you will need to create/edit the audio files, and change their volume, before you associate them to the Skype events.