You shouldn’t need to make an MP3 to find out there is no sound. You can use the Audacity sound meters and blue waves to make a good recording even if you can’t hear what it’s doing.
This is what a normal stereo recording looks like. You use the sound meters without making a recording by right-clicking on the recording sound meters > Start Monitoring. They will wake up and show you the sound—if you have any.
Modern computers no longer have Stereo Mix for recording sound foldback. That job has been taken over by WASAPI and this manual entry.
In the first (Host) box of Device Toolbar, choose “Windows WASAPI”.
In the second (Recording Device) box, choose the (loopback) input for the device you are using for playback. For example, if you are using “Speakers” for listening to playback, choose “Speakers (loopback)” in this step.
In the third (Recording Channels) box, choose mono or stereo.
In the fourth (Playback Device) box, choose the Speakers or Headphones that you are using for listening.
Start playing the audio you want to record.
Press the big red Record button in Audacity.
Of course, we assume you have copyright holders’ permission to make these recordings.