If you want a quick and easy way to stream music from your computer or phone to your home stereo and have an old Bluetooth headset lying around, Instructable user dex3844 has a simple guide to hack a stereo jack into it.
To follow the guide, all you need is a Bluetooth headset, stereo jack, soldering iron, hot glue gun and a thin wire. It's a simple, nine-step process that only requires you to pop open the casing of your headset and solder a couple wires. His tutorial guides you through a mono installation, but if you have a stereo headset the difference should only be a single wire.
There are commercially available audio dongles that do the same thing, but if you're looking for a way to do it yourself on the cheap, you can find the full tutorial over a Instructables.
Bluetooth Headset Hack [Instructables]