Infer `acodec` for single-codec containers

test/test_YoutubeDL.py

@@ -183,7 +183,7 @@ class TestFormatSelection(unittest.TestCase):
         ]
 
         info_dict = _make_result(formats)
-        ydl = YDL({'format': 'best'})
+        ydl = YDL({'format': 'best', 'format_sort': ['abr', 'ext']})
         ydl.sort_formats(info_dict)
         ydl.process_ie_result(copy.deepcopy(info_dict))
         downloaded = ydl.downloaded_info_dicts[0]
@@ -195,7 +195,7 @@ class TestFormatSelection(unittest.TestCase):
         downloaded = ydl.downloaded_info_dicts[0]
         self.assertEqual(downloaded['format_id'], 'mp3-64')
 
-        ydl = YDL({'prefer_free_formats': True})
+        ydl = YDL({'prefer_free_formats': True, 'format_sort': ['abr', 'ext']})
         ydl.sort_formats(info_dict)
         ydl.process_ie_result(copy.deepcopy(info_dict))
         downloaded = ydl.downloaded_info_dicts[0]

yt_dlp/YoutubeDL.py

@@ -2813,6 +2813,9 @@ class YoutubeDL:
             format['url'] = sanitize_url(format['url'])
             if format.get('ext') is None:
                 format['ext'] = determine_ext(format['url']).lower()
+            if format['ext'] in ('aac', 'opus', 'mp3', 'flac', 'vorbis'):
+                if format.get('acodec') is None:
+                    format['acodec'] = format['ext']
             if format.get('protocol') is None:
                 format['protocol'] = determine_protocol(format)
             if format.get('resolution') is None: