work around sometimes prob with 2-pass encoding

using mp4 muxer in place of null and not playign audio games seems to help

pack-vid

@@ -142,18 +142,20 @@ function convert( $src, $dest, $options ) {
     }
     $vf = implode( ',', $filters );
 
-    run( 'ffmpeg', [
-        '-i', $src,
-        '-f', 'null',
-        '-vf', $vf,
-        '-c:v', 'libx264',
-        '-b:v', $bitrate,
-        '-preset', 'veryslow',
-        '-pass', '1',
-        '-g', $keyframeInt,
-        '-an',
-        '-y', '/dev/null'
-    ] );
+    run( 'ffmpeg',
+        array_merge( [
+            '-i', $src,
+            '-f', 'mp4',
+            '-vf', $vf,
+            '-c:v', 'libx264',
+            '-b:v', $bitrate,
+            '-preset', 'veryslow',
+            '-pass', '1',
+            '-g', $keyframeInt,
+        ], $audio, [
+            '-y', '/dev/null'
+        ] )
+    );
     run( 'ffmpeg',
         array_merge( [
             '-i', $src,