At times I use a laptop with backing midi files to accompany myself when I play an acoustic piano. An external sound module is a good idea but can be a hassle dealing with the power/midi/audio cords. If you have a late model laptop with plenty of horsepower then you could consider using a software synthesizer such as the Roland Virtual Sound Canvas (VSC) or the Yamaha SYXG-50, and just hookup your computer to a speaker system. Both Roland and Yamaha have downloadable trial versions of these programs.