Details for .mov file: I edited the whole project in DV PAL but the files I feed into Video Converter are made using the Quick Time LAN/Intranet preset in Final Cut batch export: 640*480px, H.264, AAC audio 2 channels 48kHz, 25fps
Settings for Flash Video Converter: flash 8, 700kbps video, on2vp6 codec, 12fps, 128kbps MP3 audio.
The .mov file from which I made the failed .flv above works fine (no sound issues there). Exporting this .mov to flash 8, 400kbps video, on2vp6 codec, 12fps, 128kbps MP3 audio resulted in the same audio failure at 1:30. Exporting the audio only without video encoding from the same .mov to .flv worked just fine.
I can't see anything unusual about the movie at 1:30 when taking a look at the edits in the FCP time line.
I fixed another sequence with the same issue by filling an audio gap that my edits had left around the point where the audio later failed in the .flv - but there are no such gaps at all in the example above.
In short, I have no clue what causes these audio failures (one third of my videos are affected) and since the shortest affected sequence is 5'35'', I'd lose a lot of time if I were to systematically test different settings for the h.264 export and the subsequent .flv encoding.