Is it possible to change the pitch of your voice naturally up or down (no robot voices, etc) in real-time… with no latency? Even if using an external interface or device?
Changing the voice pitch with no theatrical damage is very rough in post production. Doing it in real time requires expensive analog processing. That and Audacity doesn’t do anything in real time except record, play, and certain timer functions.
Real time, yes.
No latency, no.
Even the best pitch shifters have some latency, though the latency can be small (less than 20 ms). There is generally a trade-off between sound quality and latency.
TC-Helicon (amongst others) have a range of vocal processor units, some of which can change the pitch of a voice with reasonable quality and reasonably low latency.
Eventide’s Eclipse V4 has a good pitch shift effect.
These type of devices are generally beyond the “prank call” price range.