USB Pluggable speakers for Macbook Pro 13" Late 2011?
I can recommend the 'SoundSticks I' from Harman Kardon, a very good 3-way Sound System. The first Generation had an USB-Interface, the later Models had the usual Headphone Jacks. You can get it from E-Bay, usually for less than 80$.

One important thing: You cannot use them on a USB-Hub, you have to connect them directly to you machine.