That looks similar to the volume indicator in my cinnamon but it's not actually for the volume by the looks of it, the size shape and position's the same but I don't know what you'd call the one on your desktop. I didn't know there were any other notifiers like this for anything else, be interesting to find out what it is . . . . . sorry i can't help though !
Lenovo ThinkPad T61 LM16-64 bit Intel T7500/2.2GHz/Cinammon 1.8 Intel GM965. Toshiba Satellite M70: LM16-32bit. Desktop:LM13 Maya 64 bit, on new Intel 3.2ghz proc/asus MB/8gb RAM