Advice for a freshman tuba player

I've only gotten to play for 2 years (we have no 6th grade in middle school), and I am now a freshman in high school and I have been working on my high range. Is there anything I can particularly work on based off of the audio? I know the High D sounds a little pinched, but I just can't get it out of my horn any other way. The sounds are much better in real life though. Any help/advice will be appreciated.