Jan. 29/14-A Mitsubishi MU-2B-60, C-FHMA, operated by Skynorth Air as flight SN692, was cruising at FL230 when the crew noted a popping sound and a pressure change. A passenger reported the #2 left cabin window had failed. The crew donned oxygen masks and carried out emergency checklists. Clearance below FL210 was not possible because of traffic, and the crew declared an emergency and descended to 14000 feet to remain above icing conditions. The flight diverted back to Winnipeg and landed without further incident. Company maintenance replaced the #2 and #3 left cabin windows and the aircraft was returned to service.