A small hole in your silencer probably won't do any damage, but the noise is offensive and probably illegal. Worse, poisonous carbon monoxide can seep into your car. Get the leak fixed as soon as possible.
Temporary repairs to small holes (under 1/8in/3mm wide) in the silencer or exhaust pipe can be made with exhaust paste (available at motor accessory shops). For larger holes (up to 1in/25mm) you can buy inexpensive heat-resistant 'bandages'. At a pinch you may be able to cover the hole temporarily by slicing open a tin and holding it in place with wire or clamps, but such a patch probably won't last more than 50 miles
