Rendering/exporting with Flash
Flash is not good at exporting (at least in my experience). It's not so fun to stay up really late finishing a project and then have to wrestle with exporting!
Here's what I do (thanks to Karl Sigler for showing me ffmpeg):
1. From Flash, export your animation as a PNG sequence. Most likely you don't want transparency, so pick "24 bit" instead of "24 bit with alpha".
2. Also from Flash, export just the soundtrack as a .WAV
3. Once that's done, join up the PNGs and WAV file like so:
ffmpeg -r 24 -i cartoon%04d.png -i cartoon.wav -b 900k cartoon.avi
"-r 24" means that your source animation is 24 frames per second. This is a good framerate to work in. Make sure this matches the number you have in your Flash file, though!
"-i cartoon%04d.png" tells ffmpeg where to find the PNG files. If you haven't done much programming the "%04d" may be confusing: it means "a four digit number padded by zeroes", and ffmpeg generates the filenames starting at 1. So in this case it would generate a series of filenames like "cartoon0001.png", "cartoon0002.png" and so on.
"-i cartoon.wav" tells ffmpeg to grab the soundtrack too.
"-b 900k" tells ffmpeg that the output movie should be 900 kbps. Depending on whether you're making a DVD or uploading to YouTube, you'll want to use different values here (the higher the number the higher the quality but the bigger the file also).
"cartoon.avi" is the output filename.