Is there an iPod touch voice control app that can play music offline?
If you're willing to sacrifice Siri for the ability to control your music with your voice, you could turn off Siri in Settings>General>Siri, which'll make your iPod respond to a held-down Home button with the classic Voice Control interface instead. A Google search is telling me that that one works offline!

For more information about non-Siri Voice Control, check out Apple's support page. Here's what it says about music playback:


  
  You can play all songs in an album, playlist, or by an artist. The device won't recognize commands to play a particular song.
  If you can't get the device to recognize the album or artist name, try telling it to play a playlist.
  If an artist has "The" in the name, say it.