As you already figured out, your sound mixer is blocking your soundsystem.
See if can kill it by doing:
(if esd is what you're using)
I'm not sure why it does work after an hour.
Flash never really worked that well for me using ALSA.
Instead, I use ALSA's OSS emulation.
To enable it type:
ALSA also has its mixer for multiple sound streams (dmix).
I find it much easier to configure than all the other ones.
Here's a good guide to configure it:
(notice you'll need to configure your apps as well for it)
Doing the above I have no problems playing any app at the same time (as for as I know).