Both are acceptable with roughly the same meaning, while "velicí" is a bit more bookish. :)
For a variety of reasons, different people may hear a word or sentence differently. Fortunately, this doesn't happen often, but it does happen. I can hear velicí on both the normal and slow recordings here, but sometimes the slow recording can be clearer.
As I can see from the comments sometimes, there is a tips and rules section, but I can't see it. Is it available on PC? I'm using duo on smartphone.
It is available on the web, including the mobile web. Select a skill and click/tap on the lightbulb.
yeah, I know about the bulb, I was using it in other languages here, but in Czech I can see it only when I use duo on my PC. Perhaps it's some kind of a system bug.
I'm using the app. I've checked the website from the smartphone and it shows the tips there, but in the app I found tips for Italian, for example, and no tips for czech. I can't attach a screenshot. Anyway, it's not a big deal, I'm just reporting to let you know about the issue.