Conditionaly compile the mux fix as it requires a new (r7593 21/01/07) or newer fffmpeg.

modules/codec/ffmpeg/mux.c

@@ -255,7 +255,13 @@ static int AddStream( sout_mux_t *p_mux, sout_input_t *p_input )
     /* This is a hack */
     if( i_codec_id == CODEC_ID_MP2 )
         i_codec_id = CODEC_ID_MP3;
-    codec->codec_tag = av_codec_get_tag( p_sys->oc->oformat->codec_tag, i_codec_id );//p_input->p_fmt->i_codec;
+#if LIBAVFORMAT_VERSION_INT >= ((51<<16)+(8<<8)+0)
+    codec->codec_tag = av_codec_get_tag( p_sys->oc->oformat->codec_tag, i_codec_id );
+#else
+#   warning "WARNING!!!!!!!"
+#   warning "Using libavformat muxing with versions older than 51.8.0 (r7593) might produce borken files.
+    codec->codec_tag = p_input->p_fmt->i_codec;
+#endif
     codec->codec_id = i_codec_id;
 
     if( p_input->p_fmt->i_extra )