A portable computer like the Amstrad PPC 640, released in 1988 has no ISA expansion ports, he has only a feeble speaker. So you may think it isnt possible to get some nice sound except of simple beeps ?
On the Amstrad CPC it was possible to have a better sound using a Digiblaster sound card connected to the parallel port. It was a simple electronic device with several resistors, not difficult to make in fact.
Well, there is the same thing on PC since ages : the Covox Speech Thing.. It is possible to buy a Covox 4 on Serdashop which is an upgraded version of the Covox Speech Thing (higher precision resistors : 0.1% and low pass filter). For more informations about the Covox, I recommand reading the related articles on Nerdly Pleasures.
I also bought the MCE2VGA on Serdashop which lets you use a VGA external screen instead of only using the internal screen which is quite lacking.