Headphones suppress system sounds

Sorry for the somewhat confusing header. I have a very strange effect on my iMac i7: if I connect headphones for watching movies during night time and put the iMac into sleep afterwards, most of the system sounds like emptying the trash or sending or receiving Mail are suppressed. If I plug in the headphones again, all suppressed sounds are played one after the other and then the iMac goes into normal operation again. Sounds of movies etc. are not concerned.