Whatever is the source of recording (sound from Windows Media Player or from an external microphone) there is a noise? or echo? Larsen?) on the records (records attached, one with no source applied).
I think in a HF oscillation because on audacity record diagram, extreme amplitudes are blue (diagram attached) and input level is not zero
How to suppress that?
I have read Doc, Forum and wiki and set
MME/Mixage stéréo (Realtek High Def)/2 stereo canaux/HP Realtek High Def
Transport/Doublage off
Transport Passage audio logiciel off
What are you trying to do? Your image shows you have Audacity set to record stereo mix, and the track is silent or very quiet (flat line).
At the moment you are in Multi-Tool which includes Envelope Tool and so you have a blue border at the top and bottom of the waveform. If you press F1 on your keyboard, you will be back in Selection Tool with no blue border.