Play ipod though laptop speakers via USB?

No, no way to do this. The USB cable doesn't support TV out or any other form of output, just charging and data transfer.

The USB cable won't do it.

You can use a Mini-Stereo Cable to connect the iPod's headset port to your laptop's audio in port. http://store.apple.com/us/product/TW027LL/A?fnode=MTY1NDA0OQ&mco=MTA4MjUwODc

If you have an Apple Universal Dock, you can connect the dock's audio line out to your laptop's audio in port. http://store.apple.com/us/product/MB125G/C?fnode=MTY1NDA0OQ&mco=MTA4NjkxNjY

You would get better sound from one of the small portable speaker systems available. They can be had for as little as US$ 49. http://store.apple.com/us/browse/home/shopipod/ipodaccessories/speakers?mco=MTM3NTAyNTI