There was a thread just last week that discussed this app and testing shutter speed generally:
Copal 0 Shutter Speed Accuracy
I don't believe that an optical assist is necessary to test to 1/30. See the first full paragraph in post #7 of that thread. It discusses Tom Johnstone's test with a lens that had just received a CLA from SK Grimes and had known shutter speeds. Unlike Johnstone, the people who claim that a sound recording isn't good enough haven't actually tried it. A search will find the full Photo.net thread.
I'd also suggest that you import your audio recording into an app that will give you a better waveform image than you'll get on a phone. For an example, see post #13.
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