#25 the sound volume should be adjustable

[Markus Koschany]

This is Debian bug http://bugs.debian.org/658769

"During the game, the sounds may be turned off/on by pressing the [S] key.
This is good, since the sounds are not always welcome.

However, I think that there should also be a way to increase/decrease
the volume of the sounds.

I hope this feature is not too difficult to implement."