| A new network or sound card is not going to increase your gaming performance noticeably. Your system configuration and video card/ motherboard/ memory play much much larger roles. I would save your money and put it towards your next upgrade to a more important component.
In regards to fixing your sound there are some wow settings that you can experiment with if you haven't already. If these settings aren't already in Warcraft\WTF\Config.wtf then you can add them to change them from the default:
SET SoundReverb "0"
Adding this I believe disables all echo effects in the game. I have this in for personal preference as the echo in Ironforge and some dungeons is otherwise unbearable.
SET SoundMaxHardwareChannels "128"
SET SoundSoftwareChannels "0"
These are my current settings for these but your mileage may vary. I believe the current default for both is 12. You could try 0 hardware and 128 or 12 software possibly. Setting both to 0 seems to result in no sound.
SET SoundOutputSystem "x"
change x to:
1 = Windows Multimedia (the one you should first try)
2 = Direct Sound
3 = Aureal3D
4 = Open Sound System
-1 = Auto select (default)
I haven't experimented with this setting personally but it could help your problem. | 