Are 'direct' derivative works of the recordings allowed?

I was trying to add voice for my game and I found Amazon Polly, and I'd like to use the voice that the service outputs. However, after reading documents such as ToS, I cannot find a definitive answer on what you can do with the recordings.

Suppose that I generated a voice recording using Amazon Polly and downloaded it as an audio file.
What I want to do with this recording is:

1. to add audio effects to the voice recording and
2. to merge Polly recordings with other audio to reduce file count.

Is it allowed to make 'direct' derivative works (I'm not sure if that is the right word) from the recording I made with Polly like this?
Or do we always have to use the recording exactly as what Polly generated?

(Note: I have all the required licenses/copyright to use the text itself to generate the recording and the audio I want to merge with the recording.)