Thank for posting! I am sorry things are not working as expected while using your UD-3900 docking station and I would be happy to help.
Thank you as well for the detail you provided and the troubleshooting you have already performed that is much appreciated.
My impressions based on your description are the following:
You have speakers (or headphones) connected to the UD-3900’s front mounted audio output jack.
Your laptop has built-in internal speakers.
The problem you describe occurs when Windows is sending audio output to the speakers connected to the dock via the “Plugable Audio” output device in Windows.
The problem you describe also occurs when Windows is sending audio output to the speakers built-in to the laptop via the “Realtek Audio” output device in Windows.
To put #3 & #4 another way, the audio problem is present and identical via both of the potential audio outputs (the laptop and the dock).
If you disconnect the UD-3900 from your laptop, the problem described in item #4 does not occur.
Is my understanding as described above correct? If not please feel free to correct me.
Assuming for the moment that my understanding is indeed correct, the behavior you describe is not expected so the next step is to get some additional information.
Does the problem occur with all audio originating from the laptop, or only audio coming from specific applications? For example, if you change the volume setting in Windows (which triggers a slight tone to indicate the volume change) does the behavior occur? Or does the problem only occur when playing back audio from a specific application? If only specific application, what application is being used?
If you would, please keep the dock connected to your system and the speakers connected to the dock (even though not working as expected) and send the output of our diagnostic utility PlugDebug --> http://plugable.com/support/plugdebug to us directly via firstname.lastname@example.org with ‘For ticket #- 292539’ in the subject line. This will allow us to match things up and help determine the next steps.
** Please also include the Amazon Order ID number for the UD-3900 purchase, it is required to confirm warranty status. **
** Please do not post the PlugDebug file or Amazon Order ID number in our public forum **
Thank you for giving us the chance to help!