This problem might derive from 3D sound positioning. Probably the sounds aren't gone, but very quiet. Maybe you aren't able to hear anything, because you scrolled out of the picture and the calculated volume is reduced too much.
- Try disabling 3D sound (EAX and DirectSound3D) in the sound options and fall back to 2D sound.
- Or maybe it's the other way round in your case. Enable DirectSound3D or EAX to hear 3D combat sounds

Furthermore, the 2D/3D Bias slider affects 3D sound volume too (left 3D sounds muted; right 3D sounds at full level).
- Try the slider at the most right position.

You can also try tweaking/checking the NWN2.ini file in your game folder

[Sound Options]
DisableSound=0

If set to 1, this option disables all sound in the game. While this may improve performance it's obviously not recommended. The main use would be for troubleshooting, to determine if crashes or other issues are sound-related for example.

Number 2D Voices=16
Number 3D Voices=16

These settings determine the number of audio channels to allocate to 2D (non-positional) and 3D (positional/environmental) sounds. The default is 16, though most sound cards can support 32. You can reduce this value to improve performance or reduce audio glitching, or conversely you can raise the value to increase the number of sounds possible. Remember that more audio channels equals more processing power required, especially for 3D sounds, so it can affect performance particularly with lots of positional audio (e.g. in heavy combat).

Speaker Type=0
This setting controls the speaker configuration used for sound playback in the game. The default option of 0 is for 2/2.1 speakers; a value of 1 is Surround, 2 is for Headphones, 3 for 4.1 speakers, 4 for 5.1 speakers, and 5 for 7.1 speakers. Set to match your speaker system or try the 2D variants (0, 1, 2)

MaxCachedSounds=100
This setting should determine how many sounds are held in the audio cache in memory. The higher the value, the more sounds are held and hence the less loading is required to play back a sound. Raising this value to 200 or even higher can help reduce stuttering in the game.