Configuring preferences and options in Outlook.com
I'm having the same issue, I noticed it sometime in the morning on Monday the 19th of August 20224. I have checked all the appropriate Outlook notification settings and web browser permissions and everything is set correctly to receive inbox notifications. I've tried different browsers and confirmed this is specific to Outlook on the web. Other web-based email inbox notifications are playing without issue. This is happening on my MS365 email account that I use for work. I work in IT and I rely on these notifications to alert me to new issues and questions from employees that require my immediate attention.
This is a serious Outlook online issue and Microsoft Support should issue a fix ASAP.