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I've been using Audacity for well over a decade now, and this has never happened before.
When I export my project to MP3 or WAV, there is no sound on playback. When I import it back into Audacity, I get waves, not a flat line. It plays just fine. I've checked the gain, I've checked the volume on the player, and have opened the exported file in multiple players.
I've checked the related posts and can't find the answer.