During a "Clean Install" I just wanted to confirm, based on my experience, the response from elstaci and add that even when the default device is connected to the HDMI, any Windows audio settings will be ignored, e.g. if normally the default sound device is set for 5.1 it will be reset to Stereo. Also if the default audio device you have chosen, is not connected to the HDMI but some other sound device is then that will indeed hijack the the other one and become a new default.
On the other hand an "Update Install", either via the Radeon Settings update option or a manual download and install on top of an existing version will preserve any/all Radeon settings you have changed and will NOT change the Windows audio setup (default or otherwise.).
To conclude an update install of the drivers will not hijack the default sound device, but BEWARE precisely because a Clean Install does reset everything it can be very useful to set a baseline and is often recommended for troubleshooting various problems.