I'm building a presentation with multiple languages and everything is working fine, but when I play it back the MP3 files sound pitched down as though there's some sort of sample rate or frame rate conversion problem. Huh? The video is playing back at 30fps and the movie is 30fps and the MP3s are synced from Final Cut. The files themselves sound fine if played back in any other app.
I'm guessing that since you've been working in Final Cut and you're saying that the MP3s are playing slow, that the sample rate on the MP3s is set to 48kHz, which is the DVD standard. Flash plays MP3s at 44.1kHz no matter what, which would explain your experience of them sounding slow. If they were done at 22kHz, they would be playing very fast.
Best bet would be to change the settings in Final Cut and re-export, but you could also use a sound editor to transcode the 48 to 44.1.
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