Fix Audio.encode_example() when writing an array
_The documentation is not available anymore as the PR was closed or merged._ @albertvillanova do you think [this line](https://github.com/huggingface/datasets/pull/3998/files#diff-58e348f6e4deaa5f3119e420a5d48ebb82875a78c28628831748fb54f59b2c78R67) is enough? that's why we missed this bug, we didn't check this case
Closes  #3996