diff --git a/audiblez/core.py b/audiblez/core.py
index 3953d4b..5bf824f 100755
--- a/audiblez/core.py
+++ b/audiblez/core.py
@@ -313,25 +313,33 @@ def create_m4b(chapter_files, filename, cover_image, output_folder):
         cover_file_path = Path(output_folder) / 'cover'
         with open(cover_file_path, 'wb') as f:
             f.write(cover_image)
-        cover_image_args = ["-i", f'{cover_file_path}', "-map", "0:a", "-map", "2:v"]
+        cover_image_args = [
+            '-i', f'{cover_file_path}',
+            '-map', '2:v',  # Map cover image
+            '-disposition:v', 'attached_pic',  # Ensure cover is embedded
+            '-c:v', 'copy',  # Keep cover unchanged
+        ]
     else:
         cover_image_args = []
 
     proc = subprocess.run([
         'ffmpeg',
-        '-y',  # overwrite output file without asking
-        '-i', f'{concat_file_path}',  # input 0 file (audio)
-        '-i', f'{chapters_txt_path}',  # input 1 file (chapters)
-        *cover_image_args,  # cover image
-        '-map', '0',  # map all streams from input 0
-        '-map_metadata', '1',  # map metadata from input 1
-        '-c:a', 'copy',  # copy audio codec
-        '-c:v', 'copy',  # copy video codec
-        '-disposition:v', 'attached_pic',  # attach cover image
-        '-c', 'copy',  # copy codec
-        '-f', 'mp4',  # format
-        f'{final_filename}'  # output file
+        '-y',  # Overwrite output
+        
+        '-i', f'{concat_file_path}',  # Input audio
+        '-i', f'{chapters_txt_path}',  # Input chapters
+        *cover_image_args,  # Cover image (if provided)
+
+        '-map', '0:a',  # Map audio
+        '-c:a', 'aac',  # Convert to AAC
+        '-b:a', '64k',  # Reduce bitrate for smaller size
+
+        '-map_metadata', '1', # Map metadata
+
+        '-f', 'mp4',  # Output as M4B
+        f'{final_filename}'  # Output file
     ])
+
     Path(concat_file_path).unlink()
     if proc.returncode == 0:
         print(f'{final_filename} created. Enjoy your audiobook.')