Apple's UI for trimming a voice memo on your iPhone is garbage. It's utterly impossible to do it, especially for audios that are over an hour long (which are great contenders for trimming). The key interactions expect my fingertip to be a pixel-wide.
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I've not used Audacity on an iPhone and the reviews there don't warrant it. I managed to somewhat do it on QuickTime Player on macOS. But yea, it looks like in this case, Apple pretty much wants you to look for other apps on the App Store to do things well.