diff --git a/abogen/tts_backend_registry.py b/abogen/tts_backend_registry.py
index 4066537..6278624 100644
--- a/abogen/tts_backend_registry.py
+++ b/abogen/tts_backend_registry.py
@@ -66,6 +66,19 @@ def register_backend(
     _registry.register(metadata, factory)
 
 
+def get_metadata(backend_id: str) -> TTSBackendMetadata:
+    """Get metadata for a specific backend by id.
+
+    Ensures all backends are registered by importing the tts_backends
+    package on first access.
+
+    Raises:
+        KeyError: If backend with given id is not registered.
+    """
+    import abogen.tts_backends  # noqa: F401  — triggers backend registration
+    return _registry.get_metadata(backend_id)
+
+
 def create_backend(backend_id: str, **kwargs: Any) -> TTSBackend:
     """Create a TTS backend instance by provider id."""
     return _registry.create_backend(backend_id, **kwargs)
diff --git a/abogen/voice_cache.py b/abogen/voice_cache.py
index fd8e4df..7d93542 100644
--- a/abogen/voice_cache.py
+++ b/abogen/voice_cache.py
@@ -17,7 +17,7 @@ if LocalEntryNotFoundError is None:  # pragma: no cover - fallback for tests
         pass
 
 
-from abogen.constants import VOICES_INTERNAL
+from abogen.tts_backend_registry import get_metadata
 
 _CACHE_LOCK = threading.Lock()
 _CACHED_VOICES: Set[str] = set()
@@ -26,8 +26,9 @@ _BOOTSTRAPPED = False
 
 
 def _normalize_targets(voices: Optional[Iterable[str]]) -> Set[str]:
+    kokoro_voices = get_metadata("kokoro").voices
     if not voices:
-        return set(VOICES_INTERNAL)
+        return set(kokoro_voices)
     normalized: Set[str] = set()
     for voice in voices:
         if not voice:
@@ -35,7 +36,7 @@ def _normalize_targets(voices: Optional[Iterable[str]]) -> Set[str]:
         voice_id = str(voice).strip()
         if not voice_id:
             continue
-        if voice_id in VOICES_INTERNAL:
+        if voice_id in kokoro_voices:
             normalized.add(voice_id)
     return normalized
 
diff --git a/abogen/voice_formulas.py b/abogen/voice_formulas.py
index b91cffe..6078d9b 100644
--- a/abogen/voice_formulas.py
+++ b/abogen/voice_formulas.py
@@ -1,7 +1,7 @@
 import re
 from typing import List, Tuple
 
-from abogen.constants import VOICES_INTERNAL
+from abogen.tts_backend_registry import get_metadata
 
 
 # Calls parsing and loads the voice to gpu or cpu
@@ -22,6 +22,7 @@ def parse_formula_terms(formula: str) -> List[Tuple[str, float]]:
         raise ValueError("Empty voice formula")
 
     terms: List[Tuple[str, float]] = []
+    kokoro_voices = get_metadata("kokoro").voices
     for segment in formula.split("+"):
         part = segment.strip()
         if not part:
@@ -30,7 +31,7 @@ def parse_formula_terms(formula: str) -> List[Tuple[str, float]]:
             raise ValueError("Each component must be in the form voice*weight")
         voice_name, raw_weight = part.split("*", 1)
         voice_name = voice_name.strip()
-        if voice_name not in VOICES_INTERNAL:
+        if voice_name not in kokoro_voices:
             raise ValueError(f"Unknown voice: {voice_name}")
         try:
             weight = float(raw_weight.strip())
diff --git a/abogen/voice_profiles.py b/abogen/voice_profiles.py
index 784b252..9823e5f 100644
--- a/abogen/voice_profiles.py
+++ b/abogen/voice_profiles.py
@@ -2,8 +2,7 @@ import json
 import os
 from typing import Any, Dict, Iterable, List, Tuple
 
-from abogen.constants import VOICES_INTERNAL
-from abogen.tts_backends.supertonic import DEFAULT_SUPERTONIC_VOICES
+from abogen.tts_backend_registry import get_metadata
 from abogen.utils import get_user_config_path
 
 
@@ -70,7 +69,8 @@ def serialize_profiles() -> Dict[str, Dict[str, Iterable[Tuple[str, float]]]]:
 
 def _normalize_supertonic_voice(value: Any) -> str:
     raw = str(value or "").strip().upper()
-    return raw if raw in DEFAULT_SUPERTONIC_VOICES else "M1"
+    supertonic_voices = get_metadata("supertonic").voices
+    return raw if raw in supertonic_voices else "M1"
 
 
 def _coerce_supertonic_steps(value: Any) -> int:
@@ -135,6 +135,7 @@ def normalize_profile_entry(entry: Any) -> Dict[str, Any]:
 
 def _normalize_voice_entries(entries: Iterable) -> List[Tuple[str, float]]:
     normalized: List[Tuple[str, float]] = []
+    kokoro_voices = get_metadata("kokoro").voices
     for item in entries or []:
         if isinstance(item, dict):
             voice = item.get("id") or item.get("voice")
@@ -143,7 +144,7 @@ def _normalize_voice_entries(entries: Iterable) -> List[Tuple[str, float]]:
             voice, weight = item[0], item[1]
         else:
             continue
-        if voice not in VOICES_INTERNAL:
+        if voice not in kokoro_voices:
             continue
         if weight is None:
             continue