Apple Mail new message sound plays inconsistently

I’m experiencing an issue where Apple Mail does not always play the “New Message” sound when an email arrives.

I’ve observed this across multiple versions of macOS (12, 15, and 26) and on four different Macs. The behaviour appears completely random—I haven’t been able to identify any pattern that explains when the sound plays and when it doesn’t.

One of the Macs running macOS 12 is a completely clean install, so this doesn’t appear to be related to leftover software, third-party utilities, or migrated settings.

Things I’ve already checked:

• “New message sound” is enabled in Mail settings
• I’ve changed the alert sound to different options (no improvement)
• System volume is up and other system sounds play normally
• The issue occurs with both a Microsoft 365 account and a separate IMAP account

At this point I’m looking for any less-obvious troubleshooting steps or insight into what might cause Mail’s notification sound to behave inconsistently.

If anyone has run into this or has deeper technical suggestions, I’d appreciate it. Please no generic or copy-and-paste responses.

Thanks.