diff --git a/synthbanshee/tts/ssml_types.py b/synthbanshee/tts/ssml_types.py
index ef44db1..78468fb 100644
--- a/synthbanshee/tts/ssml_types.py
+++ b/synthbanshee/tts/ssml_types.py
@@ -65,8 +65,14 @@ class PhraseProsody:
         char_end: Exclusive end offset.
         rate: SSML rate value (e.g. ``"+15%"``, ``"slow"``).
             ``None`` means no rate change.
-        pitch: SSML pitch value (e.g. ``"+2st"``). ``None`` means no change.
-        volume: SSML volume value (e.g. ``"+3dB"``). ``None`` means no change.
+        pitch: SSML pitch value (e.g. ``"+2st"`` or ``"+6%"``). ``None``
+            means no change. Mixing ``st`` (inner) with ``%`` (outer) is
+            tolerated by Azure.
+        volume: SSML volume value (e.g. ``"+3%"``). ``None`` means no
+            change. **Must be expressed in ``%``** to match the outer
+            ``<prosody volume="...">`` emitted by ``_volume_to_string``;
+            nesting ``volume="+NdB"`` inside ``volume="+N%"`` triggers
+            Azure SSML parser error 0x80045003 (#72).
         break_before_ms: Milliseconds of silence inserted before the span.
         break_after_ms: Milliseconds of silence inserted after the span.
     """
@@ -86,8 +92,10 @@ class PhraseProsody:
 # ---------------------------------------------------------------------------
 
 _HINT_DEFAULTS: dict[str, dict[str, str | int]] = {
-    # Stressed accusatory phrase — faster, louder, higher pitch
-    "stress": {"rate": "+15%", "volume": "+3dB", "pitch": "+1st", "break_before_ms": 0},
+    # Stressed accusatory phrase — faster, louder, higher pitch.
+    # `volume` must be in `%` (#72): Azure SSML rejects `volume="+NdB"`
+    # nested inside the outer `volume="+N%"` emitted by `_volume_to_string`.
+    "stress": {"rate": "+15%", "volume": "+3%", "pitch": "+1st", "break_before_ms": 0},
     # Deliberate command slowing — measured menace
     "slow": {"rate": "-20%", "break_before_ms": 150},
     # Pause before the phrase for dramatic weight
diff --git a/tests/unit/test_phrase_prosody.py b/tests/unit/test_phrase_prosody.py
index ce61794..6e8ebe7 100644
--- a/tests/unit/test_phrase_prosody.py
+++ b/tests/unit/test_phrase_prosody.py
@@ -15,6 +15,7 @@
 
 from synthbanshee.tts.ssml_builder import SSMLBuilder, UtteranceSpec, _apply_phrase_prosody
 from synthbanshee.tts.ssml_types import (
+    _HINT_DEFAULTS,
     PhraseHint,
     PhraseProsody,
     _build_offset_map,
@@ -106,7 +107,10 @@ def test_hint_defaults_applied_stress(self) -> None:
         )
         result = resolve_phrase_hints([hint], text, text)
         assert result[0].rate == "+15%"
-        assert result[0].volume == "+3dB"
+        # `volume` must be in `%` (#72): Azure SSML rejects `volume="+NdB"`
+        # nested inside the outer `volume="+N%"` emitted by `_volume_to_string`.
+        # Don't reintroduce `+3dB` here without also changing the outer emitter.
+        assert result[0].volume == "+3%"
         assert result[0].pitch == "+1st"
 
     def test_hint_defaults_applied_menace(self) -> None:
@@ -190,6 +194,44 @@ def test_negative_char_start_clamped(self) -> None:
         assert result[0].char_start == 0
 
 
+# ---------------------------------------------------------------------------
+# Hint defaults — Azure SSML unit invariant
+# ---------------------------------------------------------------------------
+
+
+class TestHintDefaultUnits:
+    """Azure rejects SSML when an inner `<prosody volume="+NdB">` is nested
+    inside an outer `<prosody volume="+N%">` (#72: `0x80045003 / Connection
+    was closed by the remote host`).  Since `_volume_to_string` always emits
+    the outer prosody volume in `%`, every nested phrase prosody value must
+    also be in `%`.  Reintroducing `dB` here re-breaks the corpus generation
+    for any scene that emits a `stress` hint (most violent / agitated scenes)."""
+
+    def test_no_hint_default_uses_db_for_volume(self) -> None:
+        for hint_name, defaults in _HINT_DEFAULTS.items():
+            volume = defaults.get("volume")
+            if volume is None:
+                continue
+            assert isinstance(volume, str)
+            assert not volume.endswith("dB"), (
+                f"Hint {hint_name!r} default volume {volume!r} uses dB; "
+                "must be % to match the outer prosody emitter and avoid #72 "
+                "(Azure SSML parse error 0x80045003)"
+            )
+
+    def test_hint_default_volumes_parse_as_percent(self) -> None:
+        for hint_name, defaults in _HINT_DEFAULTS.items():
+            volume = defaults.get("volume")
+            if volume is None:
+                continue
+            assert isinstance(volume, str)
+            assert volume.endswith("%"), (
+                f"Hint {hint_name!r} default volume {volume!r} must end in '%'"
+            )
+            n = volume.rstrip("%").lstrip("+")
+            int(n.lstrip("-"))  # numeric (no exception => fine)
+
+
 # ---------------------------------------------------------------------------
 # rebase_phrase_prosody
 # ---------------------------------------------------------------------------