A WiFi connection (in reference the Nintendo DS) is when the DS is connected wirelessly to your internet connection. I hope this helps!

Contrast ration is important if you want the best looking picture possible, and this is especially true if the TV is an LCD TV. The reason for this is because the larger the contrast ratio a TV has the more shades from white to black it will be able to display. I would suggest going to a Best Buy® store and taking a look at different model TV's, and choosing the one that looks best to you. I hope this helps!

Other than the Canon XSi having a larger screen, higher megapixel capability and a faster frames per second capability there are three other differences I would consider. The XSi also uses 9 points to determine auto focus, while the XS only uses 7. The XSi also has a spot meter, which is very handy in tricky lighting situations. Lastly the XSi has a greater viewfinder magnification. Both of these cameras should take great images, and it is really up to you if the added features of the XSi are worth the additional $150. I hope this helps!

An HDMI cable definitely transmits your audio signals, and you should be fine with an HDMI cable.