Problem loading voxpopuli dataset 
Version 2.18 works without problem. @Namangarg110  @mohsen-goodarzi The bug appears because the number of urls is less than 16 and the algorithm is meant to work on the previously created mode for a single url as stated on line 314: https://github.com/huggingface/datasets/blob/1bf8a46cc7b096d5c547ea3794f6a4b6c31ea762/src/datasets/download/download_manager.py#L314

In addition, previously `map_nested` function was supported without batching and it is meant to be the default performance. 

One of the shortest walk-arounds would be changing the part of the manager with the current setting:
```
        if len(url_or_urls) >= 16:
            download_func = partial(self._download_batched, download_config=download_config)
        else:
            download_func = partial(self._download_single, download_config=download_config)

        start_time = datetime.now()
        with stack_multiprocessing_download_progress_bars():
            downloaded_path_or_paths = map_nested(
                download_func,
                url_or_urls,
                map_tuple=True,
                num_proc=download_config.num_proc,
                desc="Downloading data files",
                batched=True if len(url_or_urls) >= 16 else False,
                batch_size=-1,
            )
```

I would suggest to consider other datasets for similar issues and make a pull-request.  Thanks for reporting @Namangarg110 and thanks for the investigation @MilanaShhanukova.

Apparently, there is an issue with the download functionality.
I am proposing a fix.
### Describe the bug

```
Exception has occurred: FileNotFoundError
Couldn't find file at https://huggingface.co/datasets/facebook/voxpopuli/resolve/main/{'en': 'data/en/asr_train.tsv'}
```

Error in logic for link url creation. The link should be  https://huggingface.co/datasets/facebook/voxpopuli/resolve/main/data/en/asr_train.tsv 

Basically there should be links directly under ```metadata["train"]```, not under ```metadata["train"][self.config.languages[0]]```

same for audio urls

### Steps to reproduce the bug

```
from datasets import load_dataset

dataset = load_dataset("facebook/voxpopuli","en")
```

### Expected behavior

Dataset should be loaded successfully.

### Environment info

- `datasets` version: 2.19.0
- Platform: Linux-5.15.0-1041-aws-x86_64-with-glibc2.31
- Python version: 3.10.13
- `huggingface_hub` version: 0.22.2
- PyArrow version: 16.0.0
- Pandas version: 2.2.0
- `fsspec` version: 2023.12.2