We're into the game's first expansion and now using the significantly improved DX11 client. However, we still can't switch audio devices (Ex: Speakers to USB Headset) without having to restart the client.

It's 2015, why the heck does your client forget how to playback audio when you switch default devices? Why is this annoying quirk even still around 5 years into this game's life? Have the devs really overlooked such a crucial QoL feature for so long?

Add an option in the sound settings if you have to to force the client to use your preferred device to force it to update. Any way you handle it is fine by me, as long as you handle it. This is a problem of games from 2005 and earlier, SE, it is 2015.