Whilst the TTS in DialogFlow is good, it seems like quite an important capability to use your own voice talent prompts. But using Play Audio in the responses isn't yet a feature.
I don't have a detailed/precise view on what's planned in the DialogFlow integration.
So I usually search via the PureCloud ideas lab to see which ones are committed/in development (or submitted by partners/customers).
There is nothing related to the play of audio responses (using "DialogFlow" in the search).
One idea is tagged as In development but this relates to the use of 3rd party TTS with DialogFlow. See here: https://purecloud.ideas.aha.io/ideas/CLSELF-I-274
You can request new features and share your use case at https://purecloud.ideas.aha.io/ideas . Sharing the use case is particularly important to help product management understand and prioritize the needs of our customers.