Silenzio is a free program which prevents audio devices from going to sleep a few seconds after no audio has been played.
This ordinarily does not cause a problem except that when audio plays after the sound device goes to sleep, there is noticeable latency of up to a second between the sound playing and when the device turn back on. This is unacceptable if you’re using a screen reader or doing any sort of professional audio work.
At time of writing, the latest version is 2.1. The web page is not in English so this is the direct link for the installer.
Also see SoundKeeper which seems to do the same thing.