At a guess, you have connected to the tv with a vga cable, in which case you'll need to connect a lead from the headphone or speaker out socket on the laptop to an audio in socket on the telly.
Alternatively, if you've connected using an HDMI cable, you'll probably need to go in to settings to enable audio over HDMI.
If you'd bothered to specify:
1 The laptop make/model and operating system
2 The telly make/model
3 The type of cable you are using
you could have had a definitive answer by now