But, for the Sewer System, maybe it's possible to swap the functionality of that with the Aquaduct. That is, change them so you'd need a sewer system to get past 8 and an aquaduct to get past 12. Since the aquaduct does have a unique sound, you could change that to the toilet.
Of course, that way, you can't get a unique sound for the aquaduct anymore. If you were just giving one example, but want unique sounds for all or most improvements, you're screwed.
However, if you have Test of Time, you should be able to achieve this by turning off the regular sounds and adding unique production completion sounds by events. You could do that using the CityProduction trigger, and PlayWaveFile action.