I know this is an old question, but just thought I'd add a simple solution which would be putting a load balancer (e.g. nginx) in front of your API and disabling logs for nginx (or writing to /dev/null).

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 :)