diff --git a/youtube_dl/YoutubeDL.py b/youtube_dl/YoutubeDL.py
index 702a6ad50..59c6cf75f 100755
--- a/youtube_dl/YoutubeDL.py
+++ b/youtube_dl/YoutubeDL.py
@@ -1120,11 +1120,9 @@ class YoutubeDL(object):
                 # the first that is available, starting from left
                 req_formats = rfstr.split('/')
                 for rf in req_formats:
-                    if re.match(r'.+?\+.+?', rf) is not None:
+                    if '+' in rf:
                         # Two formats have been requested like '137+139'
-                        format_1, format_2 = rf.split('+')
-                        formats_info = (self.select_format(format_1, formats),
-                                        self.select_format(format_2, formats))
+                        formats_info = [self.select_format(selected_format, formats) for selected_format in rf.split('+')]
                         if all(formats_info):
                             # The first format must contain the video and the
                             # second the audio
@@ -1139,10 +1137,8 @@ class YoutubeDL(object):
                                 else self.params['merge_output_format'])
                             selected_format = {
                                 'requested_formats': formats_info,
-                                'format': '%s+%s' % (formats_info[0].get('format'),
-                                                     formats_info[1].get('format')),
-                                'format_id': '%s+%s' % (formats_info[0].get('format_id'),
-                                                        formats_info[1].get('format_id')),
+                                'format': rf,
+                                'format_id': rf,
                                 'width': formats_info[0].get('width'),
                                 'height': formats_info[0].get('height'),
                                 'resolution': formats_info[0].get('resolution'),
@@ -1390,17 +1386,18 @@ class YoutubeDL(object):
                         postprocessors = [merger]
 
                     def compatible_formats(formats):
-                        video, audio = formats
                         # Check extension
-                        video_ext, audio_ext = audio.get('ext'), video.get('ext')
-                        if video_ext and audio_ext:
+                        video_ext = formats[0].get('ext')
+                        if video_ext:
                             COMPATIBLE_EXTS = (
                                 ('mp3', 'mp4', 'm4a', 'm4p', 'm4b', 'm4r', 'm4v'),
                                 ('webm')
                             )
                             for exts in COMPATIBLE_EXTS:
-                                if video_ext in exts and audio_ext in exts:
-                                    return True
+                                for audio_format in formats[1:]:
+                                    if video_ext not in exts or audio_format.get('ext') not in exts:
+                                        return False
+                            return True
                         # TODO: Check acodec/vcodec
                         return False
 
diff --git a/youtube_dl/postprocessor/ffmpeg.py b/youtube_dl/postprocessor/ffmpeg.py
index 1f723908b..1d8ab25a1 100644
--- a/youtube_dl/postprocessor/ffmpeg.py
+++ b/youtube_dl/postprocessor/ffmpeg.py
@@ -398,7 +398,9 @@ class FFmpegMergerPP(FFmpegPostProcessor):
     def run(self, info):
         filename = info['filepath']
         temp_filename = prepend_extension(filename, 'temp')
-        args = ['-c', 'copy', '-map', '0:v:0', '-map', '1:a:0']
+        args = ['-c', 'copy', '-map', '0:v:0']
+        for i in range(1, len(info['__files_to_merge'][1:]) + 1):
+            args.extend(['-map', '%d:a:0' % (i)])
         self._downloader.to_screen('[ffmpeg] Merging formats into "%s"' % filename)
         self.run_ffmpeg_multiple_files(info['__files_to_merge'], temp_filename, args)
         os.rename(encodeFilename(temp_filename), encodeFilename(filename))