diff --git a/gnommo/renderer.py b/gnommo/renderer.py
index fb3d089..60176c2 100644
--- a/gnommo/renderer.py
+++ b/gnommo/renderer.py
@@ -1252,9 +1252,16 @@ def build_filter_complex(
             _, first_video_source, _ = plan.narration_videos[0]
             use_channels = first_video_source.use_audio_channels
             if use_channels == "auto":
-                narration_path = _resolve_video_path(
-                    videos_dir, first_video_source, shared_assets_dir, project_path
-                )
+                # Probe the real narration audio to pick left/right/both. In concat
+                # mode that's the first processed segment; otherwise the single
+                # narration input. (plan carries the dirs — this function doesn't.)
+                if narration_concat and plan.narration_segments:
+                    narration_path = plan.narration_segments[0].source_path
+                else:
+                    narration_path = _resolve_video_path(
+                        plan.videos_dir, first_video_source,
+                        plan.shared_assets_dir, plan.project_path,
+                    )
                 use_channels = _resolve_auto_channel(narration_path)
             channel_filter = _build_audio_channel_filter(use_channels)
             narration_volume = first_video_source.volume