Hey PromptCare,
Here is a quick breakdown on the use of each of the loopbacks:
Network Loopback:
The primary purpose of this loopback is for running the external loopback test, and has the additional limitation of being unable to support gigabit speeds (which I would guess is what your system uses). Commonly it is used in environments where connecting to a network is not desirable or possible, and will simply confirm if the NIC is able to send and receive data. Also, the female connector on the adapter does not function as a passthrough, and will simply loop the data back to the connected router or switch.
Since there is a network available and yours appears to support the loopback function, I would recommend simply keeping it in reserve if necessary.
Audio Loopback:
The audio loopback cable is only intended to be used for the Rough Audio Test which does not require user interaction, and will simply send an audio signal through the cable to test the output frequencies. All other audio tests will require you to remove the loopback adapter and be able to listen to the output.
Additional information about the loopback adapters can be found in the Service Center User Guide, which can be found in the documentation folder on the DVD or USB device.
I hope this answers your questions, and please let me know if you have any others.
Thanks,
Kris