U.S.-based audio app Clubhouse appears to be blocked in mainland China after swathes of users flocked to the platform for uncensored discussions in recent weeks. An increasing number of Chinese users have gravitated to the invitation-only app, seeing it as a rare forum to express their opinions—as it does not record the conversations and thus provides a degree of privacy. Dialogues about Hong Kong’s freedoms, the suppression of Uyghurs in Xinjiang, Taiwan’s sovereignty, and other topics that Chinese censors usually deem too sensitive flourished on the app, even as Chinese users mused when Chinese authorities might come after the platform. On one popular Chinese online marketplace, prices for the app’s invite codes soared to as much as 400 yuan ($62). Netizens made similar offers on the all-in-one social media app WeChat. The burst of free expression was short-lived. On Feb. 8 evening, thousands of Chinese internet users began complaining about …