I suspect a wrong recording

Hi, I’m a newbie here and so happy you exist !

I have a question that taps me since a long time :
When I record a MP3 file from my computer, I am on Mixed Stéréo of course and all seems OK
but I suspect the music is recorded from the Loudspeaker sound by the way of Micro,
not directly from the Line. So this must be very bad, cause other noises are recorded too !

What are the correct settings to get the purest record, perfectly identical to my MP3 file ?
Thank you for any good advices.

Configuration :
Audacity 2.1.2 - loudSpeaker AltecLansing system 2.1
Desktop computer running with Windows 10 64 bits
Realtek High Definition Audio : AMD High Definition Audio Device
Play-back Périphéric : Realtek Digital Output (Realtek High Definition Audio v.1.0.64.10) (par défaut) date: 16/06/2015 ; Version: 6.0.1.7535

You are probably just recording the machine’s built-in microphone. It works, but that does record street traffic and dogs.

http://manual.audacityteam.org/man/tutorial_recording_audio_playing_on_the_computer.html

Koz