How to save the output file in a variable?

Hello, I wanted to ask about how you could save an output file that I saved in the current directory and then use it in the same program . For example, the following command extracts me a .wav file that is stored in the current directory:

system ( " sudo youtube- dl -x --audio -format wav --id https://www.youtube.com/watch?v=e4d0LOuP4Uw ');

And I want to use this .wav file later to change the sample rate and other things. How you could call or save that file ?