Sound Playback Unreliable In Windows

Hello, been reading these forums for a while, first time posting.

I have an issue with a custom built PC (Should be listed on my profile). I have reason to believe that this issue is with my software and windows, not with hardware (See below).

Relevant parts are:
Mobo: Asus P8Z77-V LK
Sound Card: Asus Xonar DG

I have been battling with this thing for several weeks now and I keep having similar issues. I will start up my machine, usually hear basic windows sounds, but then some applications will be unable to play back sound. For example, I might start Spotify to listen to music, and then open Mumble, and be unable to hear anyone speaking on Mumble (However, they will still be able to hear me). Regardless of what is going on application wise, opening the Xonar control panel and running the audio test always produces the sound in both sides of my headphones flawlessly.

I should note that I have disabled the onboard sound in my motherboard's BIOS, and that this problem happens both with the version of the Xonar drivers available from Asus's website, as well as with the version which came on the pre-packaged CD.

This issue persists when the sound card is disabled and uninstalled, the onboard sound re-enabled, and realtek drivers reinstalled, which makes me think that the sound card is fine, but that something is wrong with my motherboard or windows installation.

When I open the windows sound management and go into the "speakers" of my Asus Xonar card, and click test, I get "This device is in use by another application". Similar errors happen for the "speaker" output on the onboard audio.

The following are all things which I have tried, none of which have been successful.
Checking/unchecking the "Allow applications to take exclusive control of this device" boxes
Confirming the proper sound channels to be default for both communication and general
Disabling the systems "HD Audio Device" and uninstalling Realtek sound drivers while using Xonar DG
Uninstalling all Xonar DG drivers while using onboard sound
Updating BIOS
Updating Windows
Restarting Windows Audio and Windows Audio Endpoint Builder services (These will sometimes temporarily "reset") the problem, as will restarting the system.
Disabling startup applications
Disabling nVidia's HD Audio for HDMI
Disabling all inputs and outputs except the ones I am going to use

And of course, as microsoft would suggest, running the troubleshooter and making sure my headphones are plugged in

Any help would be greatly appreciated.

Thanks!
--NS

Asus Xonar DG and NVIDIA HDMI are the only things listed under sound. Under System Devices there is a High Definition Audio Controller. Uninstalling this just has it get reinstalled the next time I start up the machine though. Could that be the culprit?

Hard to say, I currently have it set so that nothing can take exclusive control of the sound. I'm not sure how I should go about testing if things could be monopolizing the sound, but I'll list anything that seems potentially suspicious:

VCDDaemon (For Virtual CloneDrive, a bit of software for mounting ISO's, haven't had trouble with this before).
Steam.exe (Steam Client bootstrapper)
SpotifyWebHelper.exe (Like it says)
nvvsvc.exe (NVidia driver helper service)

When I open the volume mixer, all I see is "Sound" and "System Sounds" other than master volume for speakers.

Thanks for your help!