VLC can be used quite easily to save or convert the audio of a video as MP3.
Video
Instruction
The conversion function is located in the media menu or can also be called with CTRL + R:
Now the video file needs to be added to the file selection:
Then click on “Convert/Save”.
In the next dialog, MP3 or another audio format is now selected as the profile:
A target file must also be specified. With Start the conversion starts. The conversion begins. This is not very fast. Especially with longer files it takes some time even with fast computers, because only one CPU core is used.
At some point the conversion is finished and the MP3 file can be listened to with VLC.