FFmpeg is a free and open source media file format converter. Simple usage example:

Powershell

function webmToMp3($file) {
    $ext_les_file = [io.path]::GetFileNameWithoutExtension("$file");
    ffmpeg -i "$file" -vn -ab 128k -ar 44100 -y "$ext_les_file.mp3";
    rm $file;
}

Fish shell

function webmToMp3
    set file $argv[1]
    if not string match -q "*.webm" $file
        echo "Given file is not a .webm file"
        return
    end
    # remove ext
    set ext_less_file (string replace -r '\.[^.]+$' "" $file)
    set space_less_file (string replace -a " - " "-" $ext_less_file)
    set space_less_file (string replace -a " — " "-" $ext_less_file)
    set space_less_file (string replace -a " " "_" $space_less_file)
    echo "Converting"
    echo $file
    echo "to"
    echo $space_less_file.mp3
    ffmpeg -i "$file" -vn -ab 128k -ar 44100 -y "$space_less_file.mp3"
    rm $file
end

The above function converts the provided file (in this case a .webm, hence the function name) to .mp3 format. Many more formats are supported.

Tip: Use yt-dlp to download media files from the net.