[MlMt] system alert sound randomly occurring when selecting messages in MM

[Eric Sharakan]

Hi, has anyone else noticed this odd behavior: sometime MailMate will 
emit the system alert sound when I select certain messages for viewing.  
I don't see anything unusual about the messages it chooses to do this 
with, and the behavior goes away (but eventually returns) if I restart 
MM.

I'm running r5726 on Catalina 10.15.7.

Thanks.

-Eric