Can you expand more on this? I've never heard this before, but then again most mufflers aren't installed up front. Never thought there might be a reason aside from heat.
Exhaust gasses cool off as they flow through the system, and the density of the air will change the sound. If you go turn on your hot and cold water in your house and listen you'll notice that they sound different. It's due to their differing density and the amount of friction between the water and the plumbing. Seriously.
The echo effect is also a factor- sound waves that have bounced around inside a pipe will go into a muffler sounding different than ones that just roared out of a cylinder and will therefore come out sounding much, much different. Imagine a man and woman speaking into a voice modulator- you can normally tell when the input was female even though the output is thoroughly scrambled.
Then there's the amount of pipe installed after the muffler, and that length again will dictate the final sound. I've experimented with various lengths of straight pipe attached to the same muffler on the same exhaust system and found that adding just 4" after the muffler changed the sound enough to be noticed, 8-10" was a more pronounced change, and anything 12" or longer has a drastic effect (to me). So much so that I stuck with 4" since I had no plans of running tail pipes over the axle on that setup. It's worth noting that famed Calvin Elston states that exhaust gasses require 4" of flow path to react to changes and stabilize.
When discussing exhaust with mufflers placed 2/3 or farther along in the system: longer pipes after the muffler can mellow the volume at idle as well as act like a megaphone to others at high RPM, but the tone change is not always favorable to me. Shorter or no tailpipes after the muffler typically result in a "boomier" sound across the board.
When the muffler is at the halfway point or prior then the length of the following pipe has less of an impact on the sound.
If possible, mufflers should be placed in a location that's beneficial for flow as well as optimal sound wave management and the tailpipe length can be used as a tuning aid for both performance and sound. The sound waves traveling through the pipes actually have an effect on scavenging, and some people go beyond pulse wave tuning and utilize sonic tuning.
Pulse wave tuning: We know that exhaust comes out in "pulses" and that these are positive pressure. When this positive pressure wave reaches the end of the pipe a negative pressure wave is reflected back up the pipe and adjusting the pipe length so this negative wave reaches the cylinder head at the most opportune time can maximize scavenging. A simple change of mere inches can have a measurable impact. Your typical chambered muffler flat out kills scavenging because it allows the exhaust gasses to expand quite a bit, and that slows the flow considerably since the gasses move outward in all directions instead forward towards a destination. This rapid expansion also cools the gas, which slows it down even more. If this muffler is installed early in the flow path then the rest of the system suffers by way of extremely poor flow, and poor flow will also affect the sound.
If the muffler is placed too early in the system then you're basically forcing too many sound waves of the incorrect frequency to flow though it, which is similar to sending the wrong frequency signal to your audio amplifier and cranking up the gain. It's a waste of good equipment and the outcome sounds the same: horrible.
A properly tuned performance exhaust system may utilize a venturi ring at the collector to promote flow, multiple sizes of pipe to prevent becoming a restriction to itself, and anti-reversion chambers in addition to the performance mufflers and headers that we're all familiar with. But that's a bit overkill for most.
Recap: A muffler is a restriction to flow, as is an incorrectly sized pipe, and length counts here. The length of the pipe and location of the restriction will have an effect on both the velocity as well as the tone of what comes out the end.
Hopefully my ramblings made a modicum of sense.
