Modelling room impulse by raytracing method
the impulse response is the sum of all of the different impulses received, beginning with the direct sound, then the primary reflections (only one reflection between source and receiver), then the secondary reflections (reflects offa two walls), etc.

the delay of the impulse is the delay of each path, which is the length of the path divided by the speed of sound.  each path has a path length and you get to apply the inverse-square law to that impulse delayed by that path length.  direct sound gets attenuated only by the inverse-square law.

all reflected sounds are attenuated by the inverse-square law and by each reflection.  you'll need to know something about the material of the wall at the reflection point to know how much attenuation.  a solid polished marble wall will have very little attenuation.