Heat Activated Alarm
Test the parts separately. 

1) Pull the 555 pin 4 to +V with a 1 kilohm resistor. Does the noise start? If not, investigate the 555 in more detail. But assuming yes:

2) Pull the sensor transistor base to 0V (likewise). Does it sound? If not, check the transistor and its connections. If yes:

3) Measure Q1 base voltage at different temperatures. If it increases with temperature, you have the wrong type of thermistor... To fix, interchange the thermistor with the variable resistor.  Find the base voltage at which the sounder starts to work, and adjust R (the variable R) until you get that voltage as you approach the right temperature.

If you're still stuck, add these results to the question...