added mono as option with 64k bitrate for mono

anime_audio_helper.py

@@ -82,14 +82,16 @@ def convert_audio_track(index, ch, lang, audio_temp_dir, source_file, should_dow
     else:
         # Not downmixing (or source is already stereo or less).
         # Base bitrate on the source channel count.
-        if ch == 2:      # Stereo
+        if ch == 1:      # Mono
+            bitrate = "64k"
+        elif ch == 2:    # Stereo
             bitrate = "128k"
         elif ch == 6:    # 5.1 Surround
             bitrate = "256k"
         elif ch == 8:    # 7.1 Surround
             bitrate = "384k"
-        else:            # Mono or other layouts
-            bitrate = "96k" # A sensible default for mono.
+        else:            # Other layouts
+            bitrate = "192k" # Fallback
 
     print(f"    - Encoding Audio Track #{index} to Opus at {bitrate}...")
     run_cmd([