The speed of sound changes due to pressure and temperature and humidity.
- pressure: = more molecules in a same room => they will bump into each other more often => faster propagation
- temperature = molecules moving faster => they will bump into each other more often => faster propagation
- humidity = more (water) molecules in same room => they will bump into each other more often => faster propagation
So to calculate the temperature from speed of sound you must at least also measure pressure and humidity to make a reliable reading.
For some of the math involved, see - http://www.rane.com/pdf/eespeed.pdf -