Well, I have found a way to add the Custom Field to the Content part using get_post_meta & the name of my Custom Field (‘speaker’). If it can help anyone, or if anyone wants to give me input to improve it, please do :
/*  Filter to add Custom Fields to Masonry Post list */
add_filter("avf_masonry_entry_content", function($content, $entry, $config) {
    $speakers = get_post_meta( $entry["ID"], 'speaker', true);
    $separator = ' ';
    $output = '';
   if ( ! empty( $speakers ) ) {
        $output = '<b><em>' . esc_html( $speakers ) . '</b></em><br>';
        $content = $output . $content;
    } 
    return $content;
}, 10, 3); 