audiofolder only creates dataset of 13 rows (files) when the data folder it's reading from has 20,000 mp3 files. 
### Describe the bug

x = load_dataset("audiofolder", data_dir="x")
When running this, x is a dataset of 13 rows (files) when it should be 20,000 rows (files) as the data_dir "x" has 20,000 mp3 files. Does anyone know what could possibly cause this (naming convention of mp3 files, etc.)


### Steps to reproduce the bug

x = load_dataset("audiofolder", data_dir="x")


### Expected behavior

x = load_dataset("audiofolder", data_dir="x") should create a dataset of 20,000 rows (files).

### Environment info

- `datasets` version: 2.9.0
- Platform: Linux-3.10.0-1160.80.1.el7.x86_64-x86_64-with-glibc2.17
- Python version: 3.9.16
- PyArrow version: 11.0.0
- Pandas version: 1.5.3
Hi!

> naming convention of mp3 files

Yes, this could be the problem. MP3 files should end with  `.mp3`/`.MP3` to be recognized as audio files.

If the file names are not the culprit, can you paste the audio folder's directory structure to help us reproduce the error (e.g., by running the `tree "x"` command)? Hi! I'm sorry, I don't want to reveal my entire dataset, but here's a snippet (all of the mp3 files below are some of the ones not being recognized by audiofolder. Also, for another dataset, audiofolder loaded zero mp3 files because "train" was in the name of one of the mp3 files. 
my_dataset
├── data
│   ├── VHA_Innovation_Stories_-_Day_2-123.mp3
│   ├── VHA_Innovation_Stories_-_Day_2-124.mp3
│   ├── ASSOCIATION_OF_GENERAL_PRACTITIONERS_OF_JAMAICA_NEPHROLOGY_CONFERENCE_-_JULY_3,_2022-93.mp3
│   ├── ASSOCIATION_OF_GENERAL_PRACTITIONERS_OF_JAMAICA_NEPHROLOGY_CONFERENCE_-_JULY_3,_2022-94.mp3
│   ├── ASSOCIATION_OF_GENERAL_PRACTITIONERS_OF_JAMAICA_NEPHROLOGY_CONFERENCE_-_JULY_3,_2022-95.mp3
│   ├── Your_Impact\357\274\232_Neurosurgery_equipment-5.mp3
│   └── Your_Impact\357\274\232_Neurosurgery_equipment-6.mp3
└── metadata.csv

Here's a few of the 13 files recognized by the dataset:
British_Heart_Foundation_-_Your_guide_to_a_Coronary_Angiogram,_a_test_for_heart_disease-1.mp3
British_Heart_Foundation_-_Your_guide_to_a_Coronary_Angiogram,_a_test_for_heart_disease-2.mp3
British_Heart_Foundation_-_Your_guide_to_a_Coronary_Angiogram,_a_test_for_heart_disease-3.mp3
IVP_⧸_IVU_test_Procedure_for_Kidneys_intravenous_pyelogram_-_medical_radiology_X-ray_ivp-1.mp3
IVP_⧸_IVU_test_Procedure_for_Kidneys_intravenous_pyelogram_-_medical_radiology_X-ray_ivp-2.mp3