1) "sounds like" you need to either turn down the audio level from the source (the wii?), or you need to turn the capture levels down from your capture software.
2) Your lag is built in to the capture device's own hardware encoding algorithm, and it can't be disabled. If you want to be able to see your video in "real time", then you need to "split" the audio and video out from your Wii and send it BOTH to the TV and the video capture device. (If you're lucky, your TV may even have audio out, and you could use THAT to set the capture audio level for your video capture device). In theory...
BittMann