When my Mac goes to sleep, and I awake it, AirFoil no longer detects my Airport Expert speakers.
I always need to restart OS X to make AirFoil detect it; restarting AirFoil doesn't fix it.
Do you know how could I avoid this?
I am not sure, but does Airfoil have a Helper application running in the background? Maybe you have to quit that one as well instead of just restarting AirFoil?
Also, do you have the latest version? I have just checked and the Release Notes on the Rogue Amoeba website have a bugfix in the latest release 4.6.2 from a few weeks ago that seems to match your problem:
Maybe that update can solve your problem?