If we don't have an audio/lip file for a response, how can we stop it disappearing quickly after it appears?

Yeah, this appears to be an issue. Hopefully someone can figure something out. :/

A "Dummy" (Without any audio) audio file.

Or... Wait for a SKSE Plugin to do that for you.

Create a silent wav file. I'd suggest a few different lengths (5 second, 10 second, 15 second) for different length texts. If you're unsure how to do this, then get a program like audacity, and record a microphone that has been muted for input. There are also silence files online for download.

That's kind of annoying but it will have to do.