The purpose of this test is to perform a simple measurement but also to check the frequency response and impulse response of the sound card.
You need a cable to connect the sound card input to the sound card output as shown in the figure below (except if you have a sound card that comes with a cable, such as the Digigram VXpocket).
In the figure above, a mini-jack cable is displayed which is used for most built-in and low-cost sound cards.
Setting analog input and output level