Can't vouch for using an actual microphone to time my shutter, but after reading numerous web how-to's on making an shutter tester using a phototransistor connected to the the microphone port of a computer sound card (then recording & reading out shutter speeds with a freeware sound editor like Audacity), I spent all of an afternoon and less than $20 to build one.
The difference of the old-increment shutter speeds versus new is a mere squidget compared to the nearly a whole stop sluggishness I was seeing at speeds longer than 1/25, until I gave the old Rapax shutter several Ronsonol soaks and a re-lube. At the speeds care about, it's now within 1/6 of a stop, consistently.
Easily earned back the cost of the tester in 4x5 film savings with the first round of testing.
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