At a guess you have another sound device and flash is playing the audio out of it rather then the device you wanted. You can either try and blacklist whatever other sound device you have or configure asound to make your usb device the global default (http://alsa.opensrc.org/.asoundrc#Default_PCM_device
Thanks a lot my friend. Creating the .asoundrc file was the solution to the problem.
But let me write one peculiar behaviour just for reference: after having created the /home/user/.asoundrc file I rebooted the system and I had sound through the USB speakers when using flash but only through the built-in speakers when using a KDE application such as Amarok or Kaffeine. In the multimedia section of systemsettings I noticed that the USB sound device was still there but in faint letters. I removed .asoundrc and fired up Amarok which played through the USB speakers. Then I put back .asoundrc to its place and since then I have sound through USB speakers when they are connected or the built-in speakers when no USB is present - in both flash and KDE sound applications.