To repair the microphone on a Nintendo DS Lite you should send the unit to an authorized repair center or technician.
You mean the Nintendo DS right... There should be a repair option if it is on warranty.
Take it to a DS repair dealer
Best Buy doesn't sell Nintendo Ds repair kits they do sell Nintendo Ds accessory kits and cleaning kits.
You should start off by checking out a relevant repair video via a good source for DS Lite repair videos (http://www.nintendorepairshop.com/). Once you figure out how to repair you need a good source for parts, which this website is good for as well.
An authorized Nintendo DS repair shop in Singapore is MaxSoft. It is located at 190 Middle Road 17-01 Fortune Centre, Singapore.
Take it to a DS repair dealer
Yes, if you have a coverage plan. It depends.
10-20
Internet
yes you can
Send it in to nintendo for repairs