Having witnessed 'quite a number'
of dyno tests to optimize exhaust tuning on four stroke motorcycles, I have to say I was quite amused to read you can't tune an exhaust system for a four stroke engine. Take a look at the megaphone on a single cylinder 1950s racing motorcycle, or the tuned pipe lengths of any of today's factory supplied multi-cylinder motorcycle exhausts. They don't make some of the pipes longer just because they have extra tubing laying around
Aircraft engines as mentioned by others are close to fixed speed engines, which means the exhaust would be easier to tune. The low rpm might result in relatively long pipe lengths, but you have to do the mathematics to figure it out. I'm sure the reason they didn't tune the exhaust on wartime Merlins in the modern fashion was because there was no space for the pipes.
It is interesting how the pipe configuration can alter the sound and noise level. A friend has a six-cylinder CAP aerobatic plane with a tuned system from the factory. Its remarkably quiet and sounds like an inline six racing car. Also interesting because its made from thin-wall inconel to save weight.