This code is handling the case where there is a legit gap in streaming audio, such as the following example:
sound_gap.zip
With these changes, the second audio clip will play too soon, immediately after the first clip.

As you note, high framerate movies will often get SoundStreamBlock tags clumped together into alternating frames, but you can also have gaps in the track itself, which will also appear as a large region missing SoundStreamBlock tags. I'm not 100% sure how Flash handles or differentiates these cases, hence the uncertainty in the original comments.

Perhaps this should only stop the sound if there are several (2? 3?) frames without a SoundStreamBlock, instead of a single frame.

Flash itself also does weird/buggy things when this occurs. For example, here's the above SWF with the stream bitrate set to 8kbps. The official Flash Player will play it with a gap, but too small of a gap -- it will play too soon and appear out of sync.
sound_gap2.zip
This specifically tends to occur with MP3 encoding.
The old-school workaround that animators used was to add a layer with a silent repeated sound, so that there were no gaps in the streaming audio.

Ah, right. So the stream is actually just like a normal sound with all the data already available and the playback starts at the first first SoundStreamBlock.
I thought the samples were enqueued and played back on the fly as the movie was being played so gaps wouldn't be a problem in this case.

I'm going to close this I guess as it's probably not fixable without changing how streams work :(