From: Timo Teräs

Posted upstream, see:
https://trac.videolan.org/vlc/ticket/12122

diff -ru vlc-2.1.5.orig/modules/audio_filter/channel_mixer/trivial.c vlc-2.1.5/modules/audio_filter/channel_mixer/trivial.c
--- vlc-2.1.5.orig/modules/audio_filter/channel_mixer/trivial.c	2013-06-24 21:00:38.000000000 -0300
+++ vlc-2.1.5/modules/audio_filter/channel_mixer/trivial.c	2014-09-12 16:16:49.288413334 -0300
@@ -133,7 +133,9 @@
         {
             p_src++;
         }
-        if ( p_filter->fmt_out.audio.i_physical_channels == AOUT_CHAN_CENTER )
+        if ( p_filter->fmt_out.audio.i_physical_channels == AOUT_CHAN_CENTER ||
+             p_filter->fmt_out.audio.i_physical_channels == AOUT_CHAN_LEFT ||
+             p_filter->fmt_out.audio.i_physical_channels == AOUT_CHAN_RIGHT )
         {
             /* Mono mode */
             for ( i = p_in_buf->i_nb_samples; i--; )
diff -ru vlc-2.1.5.orig/modules/stream_out/transcode/audio.c vlc-2.1.5/modules/stream_out/transcode/audio.c
--- vlc-2.1.5.orig/modules/stream_out/transcode/audio.c	2014-04-22 11:03:57.000000000 -0300
+++ vlc-2.1.5/modules/stream_out/transcode/audio.c	2014-09-12 16:28:50.805087137 -0300
@@ -35,6 +35,11 @@
 #include <vlc_meta.h>
 #include <vlc_modules.h>
 
+static const int pi_mono_channel_maps[3] =
+{
+    AOUT_CHAN_CENTER, AOUT_CHAN_LEFT, AOUT_CHAN_RIGHT,
+};
+
 static const int pi_channels_maps[6] =
 {
     0,
@@ -167,7 +172,11 @@
     if( !id->p_encoder->fmt_in.audio.i_physical_channels
      || !id->p_encoder->fmt_in.audio.i_original_channels )
     {
-        if( id->p_encoder->fmt_in.audio.i_channels < 6 )
+        if (id->p_encoder->fmt_in.audio.i_channels == 1)
+            id->p_encoder->fmt_in.audio.i_physical_channels =
+            id->p_encoder->fmt_in.audio.i_original_channels =
+                      pi_mono_channel_maps[p_sys->i_mono_source];
+        else if( id->p_encoder->fmt_in.audio.i_channels < 6 )
             id->p_encoder->fmt_in.audio.i_physical_channels =
             id->p_encoder->fmt_in.audio.i_original_channels =
                       pi_channels_maps[id->p_encoder->fmt_in.audio.i_channels];
diff -ru vlc-2.1.5.orig/modules/stream_out/transcode/transcode.c vlc-2.1.5/modules/stream_out/transcode/transcode.c
--- vlc-2.1.5.orig/modules/stream_out/transcode/transcode.c	2013-06-24 21:00:38.000000000 -0300
+++ vlc-2.1.5/modules/stream_out/transcode/transcode.c	2014-09-12 11:58:28.988259972 -0300
@@ -99,6 +99,9 @@
 #define ACHANS_TEXT N_("Audio channels")
 #define ACHANS_LONGTEXT N_( \
     "Number of audio channels in the transcoded streams." )
+#define AMONOSRC_TEXT N_("Audio source for mono")
+#define AMONOSRC_LONGTEXT N_( \
+    "Audio source channel for mono output (0=center, 1=left, 2=right)." )
 #define AFILTER_TEXT N_("Audio filter")
 #define AFILTER_LONGTEXT N_( \
     "Audio filters will be applied to the audio streams (after conversion " \
@@ -199,6 +202,8 @@
                 ALANG_LONGTEXT, true )
     add_integer( SOUT_CFG_PREFIX "channels", 0, ACHANS_TEXT,
                  ACHANS_LONGTEXT, false )
+    add_integer( SOUT_CFG_PREFIX "monosrc", 0, AMONOSRC_TEXT,
+                 AMONOSRC_LONGTEXT, false )
     add_integer( SOUT_CFG_PREFIX "samplerate", 0, ARATE_TEXT,
                  ARATE_LONGTEXT, true )
     add_bool( SOUT_CFG_PREFIX "audio-sync", false, ASYNC_TEXT,
@@ -232,7 +237,7 @@
     "venc", "vcodec", "vb",
     "scale", "fps", "width", "height", "vfilter", "deinterlace",
     "deinterlace-module", "threads", "hurry-up", "aenc", "acodec", "ab", "alang",
-    "afilter", "samplerate", "channels", "senc", "scodec", "soverlay",
+    "afilter", "samplerate", "channels", "monosrc", "senc", "scodec", "soverlay",
     "sfilter", "osd", "audio-sync", "high-priority", "maxwidth", "maxheight",
     NULL
 };
@@ -296,6 +301,8 @@
 
     p_sys->i_channels = var_GetInteger( p_stream, SOUT_CFG_PREFIX "channels" );
 
+    p_sys->i_mono_source = var_GetInteger( p_stream, SOUT_CFG_PREFIX "monosrc" );
+
     if( p_sys->i_acodec )
     {
         if( ( p_sys->i_acodec == VLC_CODEC_MP3 ||
diff -ru vlc-2.1.5.orig/modules/stream_out/transcode/transcode.h vlc-2.1.5/modules/stream_out/transcode/transcode.h
--- vlc-2.1.5.orig/modules/stream_out/transcode/transcode.h	2014-04-22 11:03:57.000000000 -0300
+++ vlc-2.1.5/modules/stream_out/transcode/transcode.h	2014-09-12 11:50:13.548255065 -0300
@@ -31,6 +31,7 @@
     config_chain_t  *p_audio_cfg;
     uint32_t        i_sample_rate;
     uint32_t        i_channels;
+    uint32_t        i_mono_source;
     int             i_abitrate;
 
     char            *psz_af;
diff -ru vlc-2.1.5.orig/src/audio_output/common.c vlc-2.1.5/src/audio_output/common.c
--- vlc-2.1.5.orig/src/audio_output/common.c	2013-12-03 11:12:35.000000000 -0200
+++ vlc-2.1.5/src/audio_output/common.c	2014-09-12 16:26:54.821752658 -0300
@@ -107,13 +107,12 @@
     case AOUT_CHAN_LEFT:
     case AOUT_CHAN_RIGHT:
     case AOUT_CHAN_CENTER:
-        if ( (p_format->i_original_channels & AOUT_CHAN_CENTER)
-              || (p_format->i_original_channels
-                   & (AOUT_CHAN_LEFT | AOUT_CHAN_RIGHT)) )
-            return "Mono";
-        else if ( p_format->i_original_channels & AOUT_CHAN_LEFT )
+        if ( p_format->i_original_channels & AOUT_CHAN_LEFT )
             return "Left";
-        return "Right";
+        else if ( p_format->i_original_channels & AOUT_CHAN_RIGHT )
+            return "Right";
+        return "Mono";
+
     case AOUT_CHAN_LEFT | AOUT_CHAN_RIGHT:
         if ( p_format->i_original_channels & AOUT_CHAN_REVERSESTEREO )
         {