audio: change --audio-channels default back to stereo

Too many problems. Well, actually it's just Linux audio systems which
cause problems, and exclusive audio access on other platforms.

In any case, it seems you have to do some manual configuration if you
want multichannel audio output.

DOCS/interface-changes.rst

@@ -19,6 +19,9 @@ Interface changes
 
 ::
 
+ --- mpv 0.16.0 ---
+    - change --audio-channels default to stereo (use --audio-channels=auto to
+      get the old default)
  --- mpv 0.15.0 ---
     - change "yadif" video filter defaults
  --- mpv 0.14.0 ---

DOCS/man/options.rst

@@ -1093,8 +1093,8 @@ Audio
         This and enabling passthrough via ``--ad`` are deprecated in favor of
         using ``--audio-spdif=dts-hd``.
 
-``--audio-channels=<number|layout>``
-    Request a channel layout for audio output (default: auto). This  will ask
+``--audio-channels=<auto|number|layout>``
+    Request a channel layout for audio output (default: stereo). This  will ask
     the AO to open a device with the given channel layout. It's up to the AO
     to accept this layout, or to pick a fallback or to error out if the
     requested layout is not supported.
@@ -1107,9 +1107,10 @@ Audio
     lists speaker names, which can be used to express arbitrary channel
     layouts (e.g. ``fl-fr-lfe`` is 2.1).
 
-    The default is ``--audio-channels=auto``, which tries to play audio using
-    the input file's channel layout. (Or more precisely, the output of the
-    audio filter chain.) (``empty`` is an accepted obsolete alias for ``auto``.)
+    ``--audio-channels=auto`` tries to play audio using the input file's
+    channel layout. There is no guarantee that the audio API handles this
+    correctly. See the HDMI warning below.
+    (``empty`` is an accepted obsolete alias for ``auto``.)
 
     This will also request the channel layout from the decoder. If the decoder
     does not support the layout, it will fall back to its native channel layout.

options/options.c

@@ -787,7 +787,7 @@ const struct MPOpts mp_default_opts = {
     .sub_visibility = 1,
     .sub_pos = 100,
     .sub_speed = 1.0,
-    .audio_output_channels = {0}, // auto
+    .audio_output_channels = MP_CHMAP_INIT_STEREO,
     .audio_output_format = 0,  // AF_FORMAT_UNKNOWN
     .playback_speed = 1.,
     .pitch_correction = 1,