There can be two reasons for chat alert not ringing:

• If there is some issue with the headphone or speaker, you might not be able to hear the ring of the chat alert. In this case, you need to check your headphone or speaker or volume settings of your system or check ring volume in the Profile section.
• If a browser session is idle for a long time, the chat alert ringing function will become disabled. Hence, to prevent this, maximize and then minimize the browser window every 15 to 20 minutes. It will keep the session active and prevent disabling of the alerts. It is a behavior of the browser when kept idle for a long time and occurs randomly.