Divide effect volume by 3 when using the OpenAL backend

Since OpenAL 1.15 AL_GAIN has much more weight than before. That leeds
to overdriven effect samples unless the volume control is set to a very
low level. With this change volume can be set to a high level without
distorting. But there's one problem. A division by 2 is to low to rule
distortion out and by 3 the game is a little bit quieter than before. A
value of 2.5 would be optimal but is not applicable since the volume is
represented by an integer. I've choosen 3 to be aon the save side.

As a side note: This problem was very less worse on Linux than on
Windows and FreeBSD. Maybe Linux guys need to pump there volume up
to compensate this work-around.
Update Windows openal32.dll to version 1.15.1