If we're sending text, then it can prove speed superintelligence by, e.g. simply answering many complicated questions all sent at once.
If we have to use voice, we can still try to ask hard questions and get fast answers, but because of the lower rate it's hard to push far past human limits. I can ask some pretty darn hard trivia questions with unambiguous answers, but someone with very good googling skills might be able to fool me.
One cute method might be to do something superhuman with the transmitted audio. For example, if it could tell how a die fell by listening to the sound of it rolling, read off handwritten digits from the faint sounds of writing, or typed letters from the acoustics of the keyboard.
In the latest William Gibson novel Agency, a bunch of human characters are constantly told exactly what to do by a superhuman AI in order to have everything turn out for the best. They accept the notion that this character, who they only talk to over their phones, is an AI, when it seems like it could just as easily be a fictional construct created by a person or organization. There's one part of the book where a character is convinced off-page that the AI is a superhuman AI by talking to it for a bit, although we're not privy to the conversation.
What I'm wondering is, how could a superhuman AI best prove that it was a superhuman AI through chat? Is there a more or less definitive way to do it? Is there a fast way to do it?