Sound and heat are related in that they are both forms of electromagnetic energy.
You need to do some reading in order to understand the concept:
http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Electromagnetic_spectrum#Infrared_radiation(scroll down to InfraRed heading)
http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Sound_wavehttp://csep10.phys.utk.edu/astr162/lect/light/spectrum.html
In the diagram above, if you were to keep going to the right beyond radiowaves, you would eventually get to a point called VF which is voice frequency, which is the frequency in which humans talk and hear at (20hz-20,000hz).
IR (InfraRed energy), or more specifically, Mid-InfraRed energy (30 THz to 120 THz) is known as 'heat' to us. When these mid-IR rays are bouncing around inside the exhaust pipe (the medium in which the sound will travel), it will change the characterisctics of the sound traveling through it.
See this page:
http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/Speed_of_soundSound travels differently based on temperature. Under high heat, soundwaves travel faster, thus making the higher pitch frequencies (i.e. rasp) bounce around with faster frequency early on in the exhaust pipe and thus their frequency widens to lower frequency within the pipe before it even makes it to the outlet of the pipe (sound waves always widen as they travel distances and bounce around off things- thus lowering the frequency in hz). So for this reason, many high pitched sounds are dissipated inside the pipe when its hot.
This is the best explaination I can come up with to try and explain this concept while still keeping it in simple enough terms so it will be understood