Mike is trying to connect his Echo to his Yamaha home stereo system, as well as his computer speakers. But when he starts playing it, it disconnects and plays on the Echo speaker. Leo says he'll have to set it up with the Echo app. The trick is to use the right command. He should be specific. For instance, he could tell it "Echo, play music in the living room."
The problem could be with Bluetooth, as the Dot may just drop the Bluetooth connection. The chatroom says that multi-speaker audio isn't supported with the Echo, so he'd have to hardwire it. Doctor Mom, though, says that works. She says the system may need repair. Hardwiring it may be the solution. Or he can just buy more Echos when they're on sale. This article at Tom's Guide could help.