process .opus files (for Multilingual Spoken Words)
Opus files requires `libsndfile>=1.0.30`. Add check for this version and tests.


**outdated:**
Add [Multillingual Spoken Words dataset](https://mlcommons.org/en/multilingual-spoken-words/)

You can specify multiple languages for downloading 😌:
```python
ds = load_dataset("datasets/ml_spoken_words", languages=["ar", "tt"])
```

1. I didn't take into account that each time you pass a set of languages the data for a specific language is downloaded even if it was downloaded before (since these are custom configs like `ar+tt` and `ar+tt+br`. Maybe that wasn't a good idea?
2. The script will have to be slightly changed after merge of https://github.com/huggingface/datasets/pull/3664 
2. Just can't figure out what wrong with dummy files... 😞 Maybe we should get rid of them at some point 😁