Also, don't use the file extension for the font-family name. It shouldn't really matter what you use, but it must match -- 
exactly -- the font-family name of the paragraph class you're trying to assign it to.
You're using:
	Code:
	font-family:"NotoSansSymbols2-Regular.ttf"
 for the @font-face statement, and:
	Code:
	font-family:"NotoSansSymbols2-Regular"
 for the CClock class.
Use:
	Code:
	font-family:"NotoSansSymbols2-Regular"
 for both.
Your src url is fine (as long as the fonts have actually been added to the Sigil epub).