The Sound tab controls the configuration of the Direct Sound output device. A scene can have multiple sound sources in different places, which have to be mixed together.

Number of active sounds:

Controls the maximum number of active sounds. If a scene tries to play more than this sounds at the same time, the least important ones are muted. Default is 16, set to 32 for modern sound cards.

HZ:

 The HZ option specifies the sound device ouput sampling rate. A higher sampling rate results in better audio quality, but requires more computing power either on the sound card or the main processor. Selecting <Default> makes BS Contact use default setting of the operating system.

Channels:

Specifies the number of output channels of the sound device.
1 is mono, 2 is stereo, 4 is surround sound, 6 is 5.1 output , 8 is 7.1. 

Selecting <Default> makes BS Contact use the default setting of the operating system. This default can be set in the speaker configuration in "Sounds and Audio Devices" in the Windows control panel. 

3D alogrithm:

Specifies the effort the computer should make for audio virtualization. Virtualization means that the sound being played is filtered in order to make it appear to originate from the same direction and distance as it is placed in the scene. This setting is a trade-off between quality and CPU utilzation.
Selecting <Default> makes BS Contact use default setting of the operating system.

No 3D: turns off 3D audio computation

For description of the other algorithms please refer to the Direct Sound Documentation.