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.