After looking into this some more, it seems that there are two problems: 1) When using the 32-bit flash plugin, there's no sound because my system lacks 32-bit pulseaudio client libraries. 2) With the 64-bit flash plugin, some pages work with the ALSA plugin whereas others crash the browser with 'illegal instruction' (this isn't related to audio -- Adobe have used an instruction that many x86_64 processors don't have). It does still appear to be the case that Flash doesn't directly use pulseaudio, but goes via ALSA. - Peter.
