diff --git a/backend/config.py b/backend/config.py
index 8d2df51a..cb6bc168 100644
--- a/backend/config.py
+++ b/backend/config.py
@@ -80,6 +80,11 @@ def resolve_storage_path(path: str | Path | None) -> Path | None:
         return None
 
     stored_path = Path(path)
+    # Empty paths (e.g. failed generations) must not resolve to the data
+    # dir itself, which exists and would defeat the callers' 404 guards.
+    # Path("") is truthy, so check parts rather than the raw value.
+    if not stored_path.parts:
+        return None
     if stored_path.is_absolute():
         rebased_path = _path_relative_to_any_data_dir(stored_path)
         if rebased_path is not None:
diff --git a/backend/routes/audio.py b/backend/routes/audio.py
index 79175568..685136c7 100644
--- a/backend/routes/audio.py
+++ b/backend/routes/audio.py
@@ -34,7 +34,7 @@ async def get_version_audio(version_id: str, db: Session = Depends(get_db)):
         raise HTTPException(status_code=404, detail="Version not found")
 
     audio_path = config.resolve_storage_path(version.audio_path)
-    if audio_path is None or not audio_path.exists():
+    if audio_path is None or not audio_path.is_file():
         raise HTTPException(status_code=404, detail="Audio file not found")
 
     return FileResponse(
@@ -52,8 +52,13 @@ async def get_audio(generation_id: str, db: Session = Depends(get_db)):
         raise HTTPException(status_code=404, detail="Generation not found")
 
     audio_path = config.resolve_storage_path(generation.audio_path)
-    if audio_path is None or not audio_path.exists():
-        raise HTTPException(status_code=404, detail="Audio file not found")
+    if audio_path is None or not audio_path.is_file():
+        detail = (
+            "Generation failed; no audio available"
+            if generation.status == "failed"
+            else "Audio file not found"
+        )
+        raise HTTPException(status_code=404, detail=detail)
 
     return FileResponse(
         audio_path,
@@ -72,7 +77,7 @@ async def get_sample_audio(sample_id: str, db: Session = Depends(get_db)):
         raise HTTPException(status_code=404, detail="Sample not found")
 
     audio_path = config.resolve_storage_path(sample.audio_path)
-    if audio_path is None or not audio_path.exists():
+    if audio_path is None or not audio_path.is_file():
         raise HTTPException(status_code=404, detail="Audio file not found")
 
     return FileResponse(
diff --git a/backend/tests/test_audio_failed_generation.py b/backend/tests/test_audio_failed_generation.py
new file mode 100644
index 00000000..c85619cc
--- /dev/null
+++ b/backend/tests/test_audio_failed_generation.py
@@ -0,0 +1,164 @@
+"""
+Regression tests for GET /audio/{generation_id} on failed generations.
+
+A failed generation stores an empty ``audio_path``. Previously,
+``config.resolve_storage_path("")`` resolved to the data directory itself,
+which exists, so the route's 404 guard passed and ``FileResponse`` raised
+``RuntimeError: File at path .../data is not a file`` — a 500 instead of
+a clean 404.
+
+Usage:
+    python -m pytest backend/tests/test_audio_failed_generation.py -v
+"""
+
+import sys
+from pathlib import Path
+
+import pytest
+from fastapi import FastAPI
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker
+from starlette.testclient import TestClient
+
+# Repo root on sys.path so ``backend`` imports as a package (the audio
+# routes use package-relative imports).
+sys.path.insert(0, str(Path(__file__).parent.parent.parent))
+
+from backend import config
+from backend.database import (
+    Base,
+    Generation,
+    GenerationVersion,
+    ProfileSample,
+    VoiceProfile,
+    get_db,
+)
+from backend.routes.audio import router as audio_router
+
+
+def test_resolve_storage_path_empty_returns_none():
+    """An empty stored path must not resolve to the data dir itself."""
+    assert config.resolve_storage_path("") is None
+    assert config.resolve_storage_path(None) is None
+    # Path("") is truthy, so it must be rejected via its (empty) parts.
+    assert config.resolve_storage_path(Path("")) is None
+
+
+@pytest.fixture
+def client(tmp_path, monkeypatch):
+    """Minimal app with only the audio routes and a temp sqlite DB."""
+    monkeypatch.setattr(config, "_data_dir", tmp_path)
+    # An existing directory that a stored audio_path may wrongly point to.
+    (tmp_path / "somedir").mkdir()
+
+    engine = create_engine(
+        f"sqlite:///{tmp_path / 'test.db'}",
+        connect_args={"check_same_thread": False},
+    )
+    Base.metadata.create_all(bind=engine)
+    testing_session_local = sessionmaker(autocommit=False, autoflush=False, bind=engine)
+
+    session = testing_session_local()
+    profile = VoiceProfile(id="profile-1", name="Test Profile")
+    session.add(profile)
+
+    session.add_all(
+        [
+            Generation(
+                id="gen-failed-empty",
+                profile_id="profile-1",
+                text="failed generation",
+                audio_path="",
+                status="failed",
+                error="engine exploded",
+            ),
+            Generation(
+                id="gen-failed-null",
+                profile_id="profile-1",
+                text="failed generation",
+                audio_path=None,
+                status="failed",
+            ),
+            Generation(
+                id="gen-missing-file",
+                profile_id="profile-1",
+                text="completed but file deleted",
+                audio_path="generations/does-not-exist.wav",
+                status="completed",
+            ),
+            Generation(
+                id="gen-with-version",
+                profile_id="profile-1",
+                text="generation with a broken version",
+                audio_path="somedir",
+                status="completed",
+            ),
+            GenerationVersion(
+                id="version-dir",
+                generation_id="gen-with-version",
+                label="original",
+                audio_path="somedir",
+            ),
+            ProfileSample(
+                id="sample-dir",
+                profile_id="profile-1",
+                audio_path="somedir",
+                reference_text="sample pointing at a directory",
+            ),
+        ]
+    )
+    session.commit()
+    session.close()
+
+    app = FastAPI()
+    app.include_router(audio_router)
+
+    def override_get_db():
+        db = testing_session_local()
+        try:
+            yield db
+        finally:
+            db.close()
+
+    app.dependency_overrides[get_db] = override_get_db
+    return TestClient(app)
+
+
+@pytest.mark.parametrize("generation_id", ["gen-failed-empty", "gen-failed-null"])
+def test_failed_generation_returns_404(client, generation_id):
+    """Failed generations (empty/null audio_path) get a clean 404, not a 500."""
+    response = client.get(f"/audio/{generation_id}")
+    assert response.status_code == 404
+    assert response.json()["detail"] == "Generation failed; no audio available"
+
+
+def test_missing_audio_file_returns_404(client):
+    """A completed generation whose file vanished still 404s."""
+    response = client.get("/audio/gen-missing-file")
+    assert response.status_code == 404
+    assert response.json()["detail"] == "Audio file not found"
+
+
+def test_unknown_generation_returns_404(client):
+    response = client.get("/audio/no-such-generation")
+    assert response.status_code == 404
+    assert response.json()["detail"] == "Generation not found"
+
+
+@pytest.mark.parametrize(
+    "url",
+    [
+        "/audio/gen-with-version",
+        "/audio/version/version-dir",
+        "/samples/sample-dir",
+    ],
+)
+def test_audio_path_pointing_at_directory_returns_404(client, url):
+    """A stored path resolving to an existing directory must 404, not 500.
+
+    Guards the is_file() checks: a directory passes exists() and would
+    crash FileResponse.
+    """
+    response = client.get(url)
+    assert response.status_code == 404
+    assert response.json()["detail"] == "Audio file not found"