Is the Wi-Fi in high-speed trains in China reliable and fast enough for audio or video conferences?

It depends on the route and mobile network coverage (after all the Wi-Fi relies on 4G/5G) as well as, of course, how many people are using it at the same time.
There are HSRs that go through mountains and rough terrains with many tunnels, e.g. Sichuan or Fujian/Anhui.

Due to the fact that the train often travels very fast (> 300 km/h), the reception and speed are not very reliable. Although if they started using 5G/EUHT on certain routes, this should in principle greatly improve.

In general, for mid or low quality streaming from Chinese websites it should be enough. WeChat audio calls are perfectly fine (at least two years ago when I took them); video calls are ok-ish.

For connections to foreign servers, it may be worse. Many apps and websites are blocked in China too, so if you are using a VPN on top of it, your connection will suffer from more latency issues.

Note that audio and video conferences will be banned in “quiet” cabins; but this is a choice you can make when you book the ticket.