Unfortunately there are similar scams for many free apps.
Official Apple support for requesting refunds can be found here: https://support.apple.com/en-gb/HT204084
Regarding Audacity:
- Audacity is free software. If you are asked to pay for it, it’s fake.
- Audacity runs on macOS, Windows, and Linux. At this time there is no version of Audacity for iPhone, iPad, or Android.