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.

Careful, foreigners will have a hard time using public Wi-Fi in China. The problem is the government doesn't want people on the internet anonymously. Wi-Fi typically has an authentication system where you provide your phone number and get a message that lets you sign in. Even if you can read the Chinese the systems I have encountered only accept Chinese phone numbers.

There is a loophole, though--the phone number need not belong to the device (and the device doesn't even need a phone number--think laptop computer), someone with a Chinese phone number who trusts you can sign you in.

(Obviously, things can change, it's been a couple of years since we have been there.)

Speaking as a local Chinese, you shouldn't expect much from the free Wi-Fi service on High-Speed Railways (HSR). These Wi-Fi services, as @zhantongz said in their answer, are backed by 4G/5G networks. As a result, the latency can be anywhere from 100ms to 10+ seconds. This is definitely not going to bring your audio/video conferences to a success. In fact, these Wi-Fi services are often "just better than none" as they suffer the same issues with your cellular network, not to mention the common poor connectivity to outside mainland China for obvious reasons.

Furthermore, speaking loudly or playing your media out loud (music / videos / TikTok etc.) is generally frowned upon in public places, particularly in closed areas like train cabins. Multiple cities have banned loud media playing in subways and it's not far from carrying out the same policy in HSRs.

Given this information, it'd be wise of you to give up ideas of holding conferences on HSR.