OSS is not well designed for sharing. It's quite likely Firefox and Flash specifically have locked the default /dev/dsp that Blitz wants to use.
Try the following for a fix:
Open a terminal and type the following
sudo aptitude install alsa-oss
You could also search for alsa-oss in Synaptic.
Then make sure Firefox is closed and enter the following from a terminal
gksudo gedit /etc/firefox/firefoxrc
Look for "FIREFOX_DSP" (it should be FIREFOX_DSP=”none”) change it for
FIREFOX_DSP=”aoss”
Save the file and restart Firefox.
This should stop Firefox from hogging /dev/dsp, but won't stop other OSS apps from doing the same.
The best way to have sound playing simultaneously is to use ALSA, because even if you don't have a card that can play multiple streams at once, it has a software mixer.