I found a pair of earmuffs at Marks Work Wearhouse that are meant to be worn with hard hats. They consist of a spring frame covered with black fleece fabric. Sometimes they slip a bit, but the springs in the framework mainly keep them on my ears with the band in back underneath my helmet. Check out a store that specializes in work clothing and safety boots, they'll probably have them.
e.t.a.: here's what they look like:
http://www.canadiantire.ca/browse/pr...romSearch=true