How to use meta_query to check if a meta field has something set?

I figured it out, meta_query must be an array within an array as it is intended for advanced queries using ‘relation’.

meta_query => array (
    array (
        //'relation' => 'OR',
        'key' => 'audio_file', //The field to check.
        'value' => '', //The value of the field.
        'compare' => '!=', //Conditional statement used on the value.
    ),  
),

The conditional ‘!=’ not-equal to ” (null) returns true if a file has been uploaded.

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