You can use the bluetooth headset to a wireless portable speaker system. All you need some basic skills of electronics and some creativity.
Here in this project I used a chinese headset and my portable guitar amplifier. We can also use input of Microphone of any big speaker system or your PC/laptop to get preamplified signal. You just need to use a cable as per the socket. In lasy pic I've shown that where I am just one variety of connectors.
Also the sound quality depends upon the type of bluetooth headset.