This is the core of an Octopress plugin that lets you put music data in posts' YAML front-matter and have a link to the iTunes Music Store show up in the post.

itms_music_link.rb

require 'cgi'
require 'json'
require 'net/http'
require 'uri'

module Jekyll
  class MusicLink < Liquid::Tag

    def initialize(tag_name, contents, tokens)
      super
      @contents = contents
    end

    def render(context)
      @affiliateCode = 'secret' # Fill in yours!
      page = context.environments.first['page']
      if page['music-artist'] && page['music-track']
        music_url, music_string = getMusic(page['music-artist'], page['music-track'])
        return %(<span class='music-box'>Music: <a class='music-link' href="#{music_url}">#{music_string}</a></span>)
      else
        return %()
      end
    end

    def makeItunesTarget(artist, track)
      iTunesURL = URI("https://itunes.apple.com/search")
      iTunesParams = {
        :country => "us", :media => "music",
        :limit => "5", :entity => "musicTrack",
        :term => artist + " " + track,
      }
      iTunesURL.query = URI.encode_www_form(iTunesParams)
      return iTunesURL
    end

    def getFromItunes(iTunesURL)
      http = Net::HTTP.new(iTunesURL.host, iTunesURL.port)
      http.use_ssl = true
      http.verify_mode = OpenSSL::SSL::VERIFY_PEER

      request = Net::HTTP::Get.new(iTunesURL.request_uri)
      response = http.request(request)
      if response.code == "200"
        jsonResponse = JSON.load(response.body)
        if jsonResponse['resultCount'] == 0
          jsonResponse = false
        end
      else
        jsonResponse = false
      end
      return jsonResponse
    end

    def makeAnchorFromItunesData(iTunesJSON)
      unless iTunesJSON
        return iTunesJSON, iTunesJSON
      end

      primaryResult = iTunesJSON['results'].first
      if @affiliateCode
        urlRegex = /(https:\/\/itunes\.apple\.com\/[^?]+\?[^&]+).+/
        urlReplacement = '\1\2&partnerId=30&siteID=' + @affiliateCode
        affiliatedTrackUrl= primaryResult['trackViewUrl'].sub(urlRegex, urlReplacement)
        primaryResult['trackViewUrl'] = affiliatedTrackUrl
      end

      anchorURL = primaryResult['trackViewUrl']
      anchorString = CGI.escapeHTML("%s - %s" % [primaryResult['artistName'], primaryResult['trackName']])
      return anchorURL, anchorString
    end

    def getMusic(artist_name, track_name)
      music_url, music_string = makeAnchorFromItunesData(getFromItunes(makeItunesTarget(artist_name, track_name)))
      return music_url, music_string
    end

  end
end

Liquid::Template.register_tag('music', Jekyll::MusicLink)