diff --git a/openrouter_client.py b/openrouter_client.py
index 14e671b..fbf30b3 100644
--- a/openrouter_client.py
+++ b/openrouter_client.py
@@ -1,8 +1,10 @@
 from __future__ import annotations
 
+import asyncio
 import base64
 import json
 import os
+import tempfile
 from pathlib import Path
 from typing import Any
 
@@ -44,31 +46,74 @@ def _build_transcription_payload(audio_path: str) -> dict[str, Any]:
             "data": encoded,
             "format": _audio_format(audio_path),
         },
+        "language": "en",
     }
 
 
+async def _normalize_audio_for_transcription(audio_path: str) -> str:
+    source = Path(audio_path)
+    fd, normalized_path = tempfile.mkstemp(prefix="meeting-normalized-", suffix=".wav")
+    os.close(fd)
+
+    proc = await asyncio.create_subprocess_exec(
+        "ffmpeg",
+        "-y",
+        "-i",
+        str(source),
+        "-ac",
+        "1",
+        "-ar",
+        "16000",
+        "-c:a",
+        "pcm_s16le",
+        normalized_path,
+        stdout=asyncio.subprocess.DEVNULL,
+        stderr=asyncio.subprocess.PIPE,
+    )
+    _, stderr = await proc.communicate()
+    if proc.returncode != 0:
+        try:
+            os.remove(normalized_path)
+        except OSError:
+            pass
+        raise RuntimeError(
+            "Audio normalization failed: " + (stderr.decode("utf-8", errors="replace").strip() or f"ffmpeg exited {proc.returncode}")
+        )
+
+    return normalized_path
+
+
 async def transcribe(audio_path: str) -> str:
     """Send audio to OpenRouter's whisper model and return transcript text."""
     headers = _auth_headers()
     headers["Content-Type"] = "application/json"
 
-    async with httpx.AsyncClient(timeout=300) as client:
-        resp = await client.post(
-            f"{OPENROUTER_BASE}/audio/transcriptions",
-            headers=headers,
-            content=json.dumps(_build_transcription_payload(audio_path)),
-        )
-        try:
-            resp.raise_for_status()
-        except httpx.HTTPStatusError as exc:
-            detail = summarize_error(_safe_json(resp), fallback=resp.text)
-            raise RuntimeError(f"OpenRouter transcription failed: {detail}") from exc
+    normalized_path = await _normalize_audio_for_transcription(audio_path)
+    try:
+        async with httpx.AsyncClient(timeout=300) as client:
+            resp = await client.post(
+                f"{OPENROUTER_BASE}/audio/transcriptions",
+                headers=headers,
+                content=json.dumps(_build_transcription_payload(normalized_path)),
+            )
+            try:
+                resp.raise_for_status()
+            except httpx.HTTPStatusError as exc:
+                detail = summarize_error(_safe_json(resp), fallback=resp.text)
+                raise RuntimeError(
+                    f"OpenRouter transcription failed ({resp.status_code}): {detail}"
+                ) from exc
 
-        data = resp.json()
-        text = data.get("text", "")
-        if not text.strip():
-            raise RuntimeError("OpenRouter transcription returned empty text")
-        return text.strip()
+            data = resp.json()
+            text = data.get("text", "")
+            if not text.strip():
+                raise RuntimeError("OpenRouter transcription returned empty text")
+            return text.strip()
+    finally:
+        try:
+            os.remove(normalized_path)
+        except OSError:
+            pass
 
 
 async def summarize(transcript: str) -> str:
diff --git a/tests/test_openrouter_client.py b/tests/test_openrouter_client.py
index fc79e56..b0f3206 100644
--- a/tests/test_openrouter_client.py
+++ b/tests/test_openrouter_client.py
@@ -21,5 +21,6 @@ def test_build_transcription_payload_uses_base64_json_shape(tmp_path: Path):
     payload = _build_transcription_payload(str(path))
 
     assert payload["model"] == "openai/whisper-large-v3"
+    assert payload["language"] == "en"
     assert payload["input_audio"]["format"] == "wav"
     assert payload["input_audio"]["data"] == base64.b64encode(b"RIFFdemo").decode("ascii")