If your using Slackware current and you have you have other things installed like lamemp3 and x264 and other sound programs . You will have to rebuild the alsa-plugin that comes with slackware.
you may have to rebuild the pulseaudio also I do after every update of pulseaudio because I need support in pulse for the several audio programs.
As I stated before what comes first the chicken or the egg. When building ffmpeg it asks do you want Pulse support and the same for pulseaudio it looks for
ffmpeg support after you rebuild both pulse the second. You must rebuild alsa-plugins to handle flash and other none cubeutil play back.
If you have like me other audio software on your system you need to do this or none of your none browser play back will work flash is handled with alsa-plugin. Hope this helps.