This is a simple example of how you can upload a track. It is done with the Ruby API wrapper but should hopefully be instructive even if you are coding in other languages.

require 'rubygems'

gem 'soundcloud-ruby-api-wrapper'
require 'soundcloud'

# This needs to be configured
settings = {
  :site                => "http://api.sandbox-soundcloud.com",
  :consumer_key        => 'c0n5um3r-k3y',
  :consumer_secret     => 'c0n5um3r-53cr37',
  :access_token_key    => '4cc35570k3n-k3y',
  :access_token_secret => '4cc35570k3n-53cr37',
}
path_to_audio_file = "/absolute/path/to/audio_file.ext"

# Setup the connection to SoundCloud
consumer = Soundcloud.consumer(settings[:consumer_key], settings[:consumer_secret], settings[:site])
access_token = OAuth::AccessToken.new(consumer, settings[:access_token_key], settings[:access_token_secret])
soundcloud = Soundcloud.register({:access_token => access_token, :site => settings[:site]})

# This is where it begins:

me = soundcloud.User.find_me
puts "You are connected to SoundCloud as: '#{me.username}'"

track = soundcloud.Track.create(
  :title      => "This is a new track",
  :asset_data => File.new(path_to_audio_file))

puts "New track '#{track.title}' is available at #{track.permalink_url}"