First, determine exactly what the goal of your demo is. There are lots of reasons to show someone an MVP - maybe your product has a fair deal of complexity, so

First, determine exactly what the goal of your demo is. There are lots of reasons to show someone an MVP - maybe your product has a fair deal of complexity, so you believe that a demo of functionality will help people understand. In turn, that understanding will allow them to better answer business questions ("Does this seem like it would solve x need for you?"). Maybe you have some basic UX mechanisms planned, and you want to test whether they're intuitive. Once you know what you're looking to get out of it, focus the entire experience on answering the important questions. If there are parts of the product that are important to see for background but aren't critical to actually have the user try out in order to answer your questions, you might be able to film videos of those parts instead of building the functionality. In your case, if you need people with good diction/utterance, just try to recruit those people. If you're trying to answer the question of whether your software solves a problem under ideal conditions, create those conditions. If it turns out the answer is yes, then you can work on solving for non-ideal conditions later and run user testing with people who have heavy accents later.