Extend WordPress Audio Widget to accept HLS (.m3u8) URLs

There is a filter, wp_audio_extensions, that should allow you to add m3u8 to the whitelist of allowed file extensions. Add this code to a child theme’s functions.php file, or using a plugin like Code Snippets:

function wpse_341199_allow_m3u8_extension( $exts ) {
    $exts[] = 'm3u8';

    return $exts;
}
add_filter( 'wp_audio_extensions', 'wpse_341199_allow_m3u8_extension' );