audiofolder creates empty dataset even though the dataset passed in follows the correct structure
### Describe the bug

The structure of my dataset folder called "my_dataset" is : data  metadata.csv
The data folder consists of all mp3 files and metadata.csv consist of file locations like 'data/...mp3 and transcriptions. There's 400+ mp3 files and corresponding transcriptions for my dataset.

When I run the following:
ds = load_dataset("audiofolder", data_dir="my_dataset")


I get:
Using custom data configuration default-...
Downloading and preparing dataset audiofolder/default to /...
Downloading data files:   0%|          | 0/2 [00:00<?, ?it/s]
Downloading data files: 0it [00:00, ?it/s]
Extracting data files: 0it [00:00, ?it/s]
Generating train split: 0 examples [00:00, ? examples/s]
Dataset audiofolder downloaded and prepared to /.... Subsequent calls will reuse this data.
  0%|          | 0/1 [00:00<?, ?it/s]
DatasetDict({
    train: Dataset({
        features: ['audio', 'transcription'],
        num_rows: 1
    })
})





### Steps to reproduce the bug

Create a dataset folder called 'my_dataset' with a subfolder called 'data' that has mp3 files. Also, create metadata.csv that has file locations like 'data/...mp3' and their corresponding transcription.

Run:
ds = load_dataset("audiofolder", data_dir="my_dataset")

### Expected behavior

It should generate a dataset with numerous rows. 

### Environment info

Run on Jupyter notebook