Packing user created Bangai-O Spirits levels into modem sounds is an unnecessary, but awesome technical feat from the wizards at Treasure. To upload a level via sound transfer you need to get the sound file off your DS and on to your computer. A microphone is a handy tool for the task, but a better way (I assume) is to connect a male to male ¾ inch cord to the DS and directly into the microphone jack. Hit record, then save and the file is ready for distribution.
But where on the internet should this audio file go? Some Bangai-O Spirits players are using YouTube as the place to hold the data. This video shows an example of a “Bangai-O Tube” transfer where a user downloads a level by streaming a file from YouTube and has the DS microphone connected via headphones. Creative!