diff --git a/yt_dlp/YoutubeDL.py b/yt_dlp/YoutubeDL.py
index ea6264a0d..707578f0d 100644
--- a/yt_dlp/YoutubeDL.py
+++ b/yt_dlp/YoutubeDL.py
@@ -3452,6 +3452,7 @@ def correct_ext(filename, ext=new_ext):
 
                     merger = FFmpegMergerPP(self)
                     downloaded = []
+                    should_merge = True
                     if dl_filename is not None:
                         self.report_file_already_downloaded(dl_filename)
                     elif fd:
@@ -3469,12 +3470,20 @@ def correct_ext(filename, ext=new_ext):
                                 'You have requested merging of multiple formats '
                                 'while also allowing unplayable formats to be downloaded. '
                                 'The formats won\'t be merged to prevent data corruption.')
+                            should_merge = False
                         elif not merger.available:
                             msg = 'You have requested merging of multiple formats but ffmpeg is not installed'
                             if not self.params.get('ignoreerrors'):
                                 self.report_error(f'{msg}. Aborting due to --abort-on-error')
                                 return
                             self.report_warning(f'{msg}. The formats won\'t be merged')
+                            should_merge = False
+                        elif any(f.get('container') == 'iamf' for f in info_dict['requested_formats']):
+                            self.report_warning(
+                                'You have requested merging of multiple formats '
+                                'but one of the formats is an IAMF audio format. '
+                                'The formats won\'t be merged to prevent data loss.')
+                            should_merge = False
 
                         if temp_filename == '-':
                             reason = ('using a downloader other than ffmpeg' if FFmpegFD.can_merge_formats(info_dict, self.params)
@@ -3500,7 +3509,7 @@ def correct_ext(filename, ext=new_ext):
                             info_dict['__real_download'] = info_dict['__real_download'] or real_download
                             success = success and partial_success
 
-                    if downloaded and merger.available and not self.params.get('allow_unplayable_formats'):
+                    if downloaded and should_merge:
                         info_dict['__postprocessors'].append(merger)
                         info_dict['__files_to_merge'] = downloaded
                         # Even if there were no downloads, it is being merged only now
diff --git a/yt_dlp/extractor/youtube/_video.py b/yt_dlp/extractor/youtube/_video.py
index afbb34fcf..2bcf8d967 100644
--- a/yt_dlp/extractor/youtube/_video.py
+++ b/yt_dlp/extractor/youtube/_video.py
@@ -3489,7 +3489,7 @@ def build_fragments(f):
                 itags[itag].add(('https', dct.get('language')))
                 stream_ids.append(stream_id)
             single_stream = 'none' in (dct.get('acodec'), dct.get('vcodec'))
-            if single_stream and dct.get('ext'):
+            if single_stream and dct.get('ext') and not dct.get('container'):
                 dct['container'] = dct['ext'] + '_dash'
 
             if (all_formats or 'dashy' in format_types) and dct['filesize']:
diff --git a/yt_dlp/utils/_utils.py b/yt_dlp/utils/_utils.py
index c89f50a51..7e8acc824 100644
--- a/yt_dlp/utils/_utils.py
+++ b/yt_dlp/utils/_utils.py
@@ -3024,7 +3024,7 @@ def parse_codecs(codecs_str):
         return {}
     split_codecs = list(filter(None, map(
         str.strip, codecs_str.strip().strip(',').split(','))))
-    vcodec, acodec, scodec, hdr = None, None, None, None
+    vcodec, acodec, scodec, hdr, container = None, None, None, None, None
     for full_codec in split_codecs:
         full_codec = re.sub(r'^([^.]+)', lambda m: m.group(1).lower(), full_codec)
         parts = re.sub(r'0+(?=\d)', '', full_codec).split('.')
@@ -3042,6 +3042,8 @@ def parse_codecs(codecs_str):
         elif parts[0] in ('flac', 'mp4a', 'opus', 'vorbis', 'mp3', 'aac', 'ac-4',
                           'ac-3', 'ec-3', 'eac3', 'dtsc', 'dtse', 'dtsh', 'dtsl', 'iamf'):
             acodec = acodec or full_codec
+            if parts[0] == 'iamf':
+                container = 'iamf'
         elif parts[0] in ('stpp', 'wvtt'):
             scodec = scodec or full_codec
         else:
@@ -3052,6 +3054,7 @@ def parse_codecs(codecs_str):
             'acodec': acodec or 'none',
             'dynamic_range': hdr,
             **({'scodec': scodec} if scodec is not None else {}),
+            **({'container': container} if container is not None else {}),
         }
     elif len(split_codecs) == 2:
         return {