Nick,
On the old ICUE versions, there was an XML file that had the sidetone volume MAXED at only 25%. Editing this to 100% fixed the issue for everyone.
With the newest version of ICUE, it seems that file has vanished and the sidetone is permanently maxed at 25%
It was located in program files\corsair\Corsair Utility Engine\devices\profile_void_usb.xml
<maxSidetoneLevelValue>25</maxSidetoneLevelValue>
For all intents and purposes, the sidetone on the Void Elite Pro is practically useless now.
There is something in the software limiting the sidetone. I have searched through all of ICUE's install files and no option like this exists now.