You can get a $25 bluetooth transmitter. e.g.

But do not expect great sound quality from a $25 part. The analog to digital conversion needed before transmission over BT will color the sound, better parts cost more.

I use sennheiser wireless (not BT) headphones and like them (old model hdr 85) but really hear a difference in music with my wired headset.

Also check the RANGE on your headset. Some BT is designed for short distances.

Also check to see if your headset supports Multiple Pairing. Some headsets will only talk with one device, and if it was set to your phone it would not work with a BT braodcast. Other headsets connect to (pari with) several other devices.

EDIT: I assumed you were not using a PC to coordinate your home theater system. If you are using a PC to generate the sound then you can get a USB bluetooth adapter cheap and set your sound to output through it. Price is less important here - the USB adapter is not doing any analog to digital.