Aspeaker says, "because the japanese make the best stereo sound systems in the world, you should consider buying a japanese-made sound system." this speaker is guilty of using which kind of fallacy in reasoning?

The speaker is using the fallacy of building on an unproven assumption. The speaker has made the assumption that the Japanese make the best stereo sound systems in the world. This is merely his/her opinion, and is not a proven fact. Thus, when the speaker bases another argument on that assumption, he has used a fallacy in reasoning.