Is decreasing speaker volume of my iMac a Hardware Problem My iMac is just less than 1 year old.  I have ran the diagnostic that came with Apple Care and found no problem.  Recently, I notice that the max speaker volume of my iMac decreased after I switch on the machine on for quite some time. If I leave the iMac off a few days, the volume will sort of reset again but it is not as loud as before.  I believe this problem has nothing to do will software settings in OSX as I have tried all those settings.  Any recommendation, send the iMac back to Apple?
I recommend you make sure it’s not a perception issue and that the sound you’re using to test the volume is always the same. 

Perhaps making a short “video” with another device of both moments (when it’s high and when it’s not), might help you make sure that this is not a crazy thing on your part. If the camera is left in the same place and the conditions are more or less the same, you should be able to clearly hear both videos and see a volume difference. 

If you are sure that the volume tends to “go down” and is not a software issue, then take the machine to Apple as soon as possible, because I’ve never heard of decreasing speaker volumes. :)