How concatenate `Audio` elements using batch mapping
### Describe the bug

I am trying to do concatenate audios in a dataset e.g. `google/fleurs`.
```python
print(dataset)
# Dataset({
#     features: ['path', 'audio'],
#     num_rows: 24
# })

def mapper_function(batch):
   # to merge every 3 audio
   # np.concatnate(audios[i: i+3]) for i in range(i, len(batch), 3)

dataset = dataset.map(mapper_function, batch=True, batch_size=24)
print(dataset)
# Expected output:
# Dataset({
#     features: ['path', 'audio'],
#     num_rows: 8
# })
```

I tried to construct `result={}` dictionary inside the mapper function, I just found it will not work because it needs `byte` also needed :((

I'd appreciate if your share any use cases similar to my problem or any solutions really. Thanks!

cc: @lhoestq 

### Steps to reproduce the bug

1. load audio dataset
2. try to merge every k audios and return as one

### Expected behavior

Merged dataset with a fewer rows. If we merge every 3 rows, then `n // 3` number of examples.

### Environment info

- `datasets` version: 2.1.0
- Platform: Linux-5.15.65+-x86_64-with-debian-bullseye-sid
- Python version: 3.7.12
- PyArrow version: 8.0.0
- Pandas version: 1.3.5