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