I agree with @EbrahimKhalilHassen. You'll get better SEO, but not only that it'll be clear to users that your product is independent. If a user visited uisounds.prototypr.io, they might be confused about whether UI Sounds was its own product, and how it might relate to prototypr.io.

If you have a domain for it already, I'd definitely say use it. It also means you'll be able to send emails to users from that domain :)