A few times, I was debugging a "sound problem", worrying about hardware and driver configuration, just to find out at the end that the volume was muted.
In all applications, on a new installation, the volume should be on and to the maximum by default. People know how to turn down the volume, and at least, this way, they know that the sound works!!
This is valid for Desktop Environment:
- KDE / Plasma (kmix, etc.)
- mplayer, etc.
for the system: