I'm not sure I understand your question. What do you call a "media player"? Surely you have the built-in "Music" app to play audio. The built-in "Music" app isn't great, but it works. For music, you might want to go to the Android Market and try some of the other free music player apps, like Mortplayer, WinAmp, DoubleTwist, or BoomBoxoid. (BoomBoxoid is good if you need to play WMA files.) If you're trying to plan video files, you may want to go to the Android Market and download the free MoboPlayer or RockPlayer. So now you can play audio or video files.
Now then, how do you hear them on a Bluetooth device. That depends. First you have pair the Bluetooth device with your LG Optimus. You can look up how to do that. If it's a set of Bluetooth headphones, the music should just play through them. If it's a Bluetooth microphone earpiece, then you'll need to load an app like BTMono, which will route your stereo sound into a single earpiece.