If your midis don't specify a particular voice, your sound card will probably use the "last heard" preset as its default until another midi with a specified voice is played, so playing one with a voice you actually like would sort the problem in the short term. Re-starting the computer probably achieves a return to the usual piano default voice; I don't remember at present. If you have midi-editing software, you can tell your own files what voice to use, and they then won't be affected by other files played; you do have to change each file individually, though.