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function wp_audio_shortcode( $attr, $content = '' ) {
	$post_id = get_post() ? get_the_ID() : 0;

	static $instance = 0;
	++$instance;

	/**
	 * Filters the default audio shortcode output.
	 *
	 * If the filtered output isn't empty, it will be used instead of generating the default audio template.
	 *
	 * @since 3.6.0
	 *
	 * @param string $html     Empty variable to be replaced with shortcode markup.
	 * @param array  $attr     Attributes of the shortcode. See {@see wp_audio_shortcode()}.
	 * @param string $content  Shortcode content.
	 * @param int    $instance Unique numeric ID of this audio shortcode instance.
	 */
	$override = apply_filters( 'wp_audio_shortcode_override', '', $attr, $content, $instance );

	if ( '' !== $override ) {
		return $override;
	}

	$audio = null;

	$default_types = wp_get_audio_extensions();
	$defaults_atts = array(
		'src'      => '',
		'loop'     => '',
		'autoplay' => '',
		'preload'  => 'none',
		'class'    => 'wp-audio-shortcode',
		'style'    => 'width: 100%;',
	);
	foreach ( $default_types as $type ) {
		$defaults_atts[ $type ] = '';
	}

	$atts = shortcode_atts( $defaults_atts, $attr, 'audio' );

	$primary = false;
	if ( ! empty( $atts['src'] ) ) {
		$type = wp_check_filetype( $atts['src'], wp_get_mime_types() );

		if ( ! in_array( strtolower( $type['ext'] ), $default_types, true ) ) {
			return sprintf( '<a class="wp-embedded-audio" href="%s">%s</a>', esc_url( $atts['src'] ), esc_html( $atts['src'] ) );
		}

		$primary = true;
		array_unshift( $default_types, 'src' );
	} else {
		foreach ( $default_types as $ext ) {
			if ( ! empty( $atts[ $ext ] ) ) {
				$type = wp_check_filetype( $atts[ $ext ], wp_get_mime_types() );

				if ( strtolower( $type['ext'] ) === $ext ) {
					$primary = true;
				}
			}
		}
	}

	if ( ! $primary ) {
		$audios = get_attached_media( 'audio', $post_id );

		if ( empty( $audios ) ) {
			return;
		}

		$audio       = reset( $audios );
		$atts['src'] = wp_get_attachment_url( $audio->ID );

		if ( empty( $atts['src'] ) ) {
			return;
		}

		array_unshift( $default_types, 'src' );
	}

	/**
	 * Filters the media library used for the audio shortcode.
	 *
	 * @since 3.6.0
	 *
	 * @param string $library Media library used for the audio shortcode.
	 */
	$library = apply_filters( 'wp_audio_shortcode_library', 'mediaelement' );

	if ( 'mediaelement' === $library && did_action( 'init' ) ) {
		wp_enqueue_style( 'wp-mediaelement' );
		wp_enqueue_script( 'wp-mediaelement' );
	}

	/**
	 * Filters the class attribute for the audio shortcode output container.
	 *
	 * @since 3.6.0
	 * @since 4.9.0 The `$atts` parameter was added.
	 *
	 * @param string $class CSS class or list of space-separated classes.
	 * @param array  $atts  Array of audio shortcode attributes.
	 */
	$atts['class'] = apply_filters( 'wp_audio_shortcode_class', $atts['class'], $atts );

	$html_atts = array(
		'class'    => $atts['class'],
		'id'       => sprintf( 'audio-%d-%d', $post_id, $instance ),
		'loop'     => wp_validate_boolean( $atts['loop'] ),
		'autoplay' => wp_validate_boolean( $atts['autoplay'] ),
		'preload'  => $atts['preload'],
		'style'    => $atts['style'],
	);

	// These ones should just be omitted altogether if they are blank.
	foreach ( array( 'loop', 'autoplay', 'preload' ) as $a ) {
		if ( empty( $html_atts[ $a ] ) ) {
			unset( $html_atts[ $a ] );
		}
	}

	$attr_strings = array();

	foreach ( $html_atts as $k => $v ) {
		$attr_strings[] = $k . '="' . esc_attr( $v ) . '"';
	}

	$html = '';

	if ( 'mediaelement' === $library && 1 === $instance ) {
		$html .= "<!--[if lt IE 9]><script>document.createElement('audio');</script><![endif]-->\n";
	}

	$html .= sprintf( '<audio %s controls="controls">', implode( ' ', $attr_strings ) );

	$fileurl = '';
	$source  = '<source type="%s" src="%s" />';

	foreach ( $default_types as $fallback ) {
		if ( ! empty( $atts[ $fallback ] ) ) {
			if ( empty( $fileurl ) ) {
				$fileurl = $atts[ $fallback ];
			}

			$type  = wp_check_filetype( $atts[ $fallback ], wp_get_mime_types() );
			$url   = add_query_arg( '_', $instance, $atts[ $fallback ] );
			$html .= sprintf( $source, $type['type'], esc_url( $url ) );
		}
	}

	if ( 'mediaelement' === $library ) {
		$html .= wp_mediaelement_fallback( $fileurl );
	}

	$html .= '</audio>';

	/**
	 * Filters the audio shortcode output.
	 *
	 * @since 3.6.0
	 *
	 * @param string $html    Audio shortcode HTML output.
	 * @param array  $atts    Array of audio shortcode attributes.
	 * @param string $audio   Audio file.
	 * @param int    $post_id Post ID.
	 * @param string $library Media library used for the audio shortcode.
	 */
	return apply_filters( 'wp_audio_shortcode', $html, $atts, $audio, $post_id, $library );
}