Is the .flac audio file format supported by WordPress?

Yes, the flac audio format is supported. See the doc block for the function wp_get_mime_types() (and other mentions).

/**
 * Retrieve list of mime types and file extensions.
 *
 * @since 3.5.0
 * @since 4.2.0 Support was added for GIMP (.xcf) files.
 * @since 4.9.2 Support was added for Flac (.flac) files.
 * @since 4.9.6 Support was added for AAC (.aac) files.
 *
 * @return string() Array of mime types keyed by the file extension regex corresponding to those types.
 */
function wp_get_mime_types() {

Whether the visitor’s browser supports it is a completely different question and not in WordPress’ hands.