Who changed SQL server max memory configuration? Is there a way to tell who changed SQL server's max memory after a several SQL reboots?. I looked at the default trace log for event category 81 (memory configuration changes) but could not find anything. 
SQL server -->reports--> configuration report changes is also empty. 
Please suggest.
YES, there is a way to find out Who did it.

On SQL Log file viewer, on left panel select Windows NT to expand and select Application to display.

You can see details of date, User, computer, etc:

message:
Date        8/02/2014 
Log     Windows NT (Application)

Source      MSSQLSERVER
Category        (2)
Event       2342
User        ???????   THIS IS WHO  "OFFENDER"
Computer    THIS IS THE HOST where he did it

Message
Configuration option 'min server memory (MB)' changed from 2048 to 512. Run the RECONFIGURE statement to install.


Good luck.