iPhone as a PC speaker through USB cable

I use a PC at work and the stock sound card is garbage, so the audio quality of music when I listen to Spotify or Pandora is terrible. I have an iPhone and was wondering if there is any way to route the audio output through my iPhone for improved sound quality. I've found a number of applications that do this (WifFi2HiFi, RemoteSound, AirFoil), however all work over WiFi and my PC is not connected to a WiFi network. Is anyone aware of how I can accomplish this through the USB cable? (I realize I can use the apps directly on my phone but am trying to minimize data consumption on my plan).

Thanks for the help!