On Windows systems you will find “Loopback Output” devices in the “Windows Audio” submenu, that can be selected as inputs. This loopback is only available if your player software uses a Windows Audio driver as output. When playing back audio from a DAW software, which normally uses ASIO drivers, you can use the IQ-Series Analyser NetAux plugin to show a signal in IQ-Series Analyser (Standalone) (see chapter Network function in the manual for details).
Unfortunately, on OS X systems a direct loopback of the audio output is not possible. You need to use an additional audio routing tool to achieve this. For instance, you could use Soundflower (https://rogueamoeba.com/freebies/soundflower/) which is available for free. Of course you can also use the IQ-Series Analyser NetAux plugin with a DAW on OS X.