When you sing the music in your head while playing, your brain gives orders to your lips, fingers and the rest of your body to produce what is playing in your head. It will send signals to your muscles and after a few times playing like this, will find the most efficient way to do it. Of course you have to know your music. We very often say: "if you can sing it, you can play it" . It is true assuming that you have the strength to reach the notes of course.
I personally find after a while doing this that my lips start feeling very good and my tone improves a lot. I also have more endurance. I believe the reason for that is that the brain is acting to produce the desired notes and sounds rather than reacting to what it hears. The brains no longer needs to correct each note's tone and pitch after it hears them thus saving energy to play longer.
If you learn to play an instrument, wouldn't it be nice to hear pieces that are written originally for that instrument? And played by the world's best players on top of that! And you don't even have to buy cds anymore! You can download it or just watch some Youtube videos for free.
I said in a previous post that you should listen to other kind of music sometimes. That's assuming that you do take time to develop some knowledge about your own instrument in both solo and orchestral repertoire. It's important to know as much as you can about your instrument and that includes that instrument's repertoire. The good news is: It's fun!
So go to Youtube right now and please yourself with some nice horn or brass music. You'll love it!
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.