Is decreasing speaker volume of my iMac a Hardware Problem
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. :)