Where can I find a free program which will extract sound files from SWF files?

I can only find stuff that you've got to pay for.

Hmmm... you mean "Shockwave Flash" files? You could always capture the sound with an external recording program (like Audacity) if your soundcard supports it. Most new SBs should be able to handle such a thing, but cheap ones have sometimes such bad drivers that they aren't able to do the trick.