[Bugfix] Voxtral prompt/audio placeholder alignment (#34140)

Signed-off-by: Artus KG <artuskg@gmail.com>

vllm/model_executor/models/voxtral.py

@@ -187,6 +187,7 @@ class VoxtralProcessingInfo(BaseProcessingInfo):
     def get_data_parser(self):
         return MultiModalDataParser(
             target_sr=self.get_hf_processor().sampling_rate,
+            target_channels=1,
             expected_hidden_size=self._get_expected_hidden_size(),
         )
 
@@ -289,10 +290,24 @@ class VoxtralMultiModalProcessor(BaseMultiModalProcessor[VoxtralProcessingInfo])
         processor = self.info.get_hf_processor(**hf_processor_mm_kwargs)
 
         audio_id = processor.audio_token_id
+        out_mm_data = out_mm_kwargs.require_data()
+        out_audio_items = out_mm_data.get("audio", [])
 
         def get_replacement(item_idx: int):
-            audios = mm_items.get_items("audio", AudioProcessorItems)
-            audio_len = audios.get_audio_length(item_idx)
+            if item_idx < len(out_audio_items):
+                out_audio_data = out_audio_items[item_idx].get_data()
+                audio_arr = out_audio_data["audio_arrays"]
+                if isinstance(audio_arr, (torch.Tensor, np.ndarray)):
+                    audio_len = len(audio_arr)
+                else:
+                    raise TypeError(
+                        "Unexpected type for audio_arrays in out_mm_kwargs: "
+                        f"{type(audio_arr)}"
+                    )
+            else:
+                # Fallback for unexpected processor outputs.
+                audios = mm_items.get_items("audio", AudioProcessorItems)
+                audio_len = audios.get_audio_length(item_idx)
 
             nb_audio_tokens = processor.get_num_audio_tokens(audio_len)
 
@@ -495,7 +510,10 @@ class VoxtralForConditionalGeneration(
         return TokensPrompt(
             prompt_token_ids=tokenized.tokens,
             multi_modal_data={
-                "audio": (tokenized.audios[0].audio_array, stt_config.sample_rate)
+                "audio": [
+                    (audio.audio_array, stt_config.sample_rate)
+                    for audio in tokenized.audios
+                ],
             },
         )