A cello or viola?
Not the highest nor the lowest
With letters on my chest
Thanks to d'alar'cop for helping me with these two lines!
A cello and viola are between the violin (highest) and the double bass (lowest). Both cellos and violas have what look like letter S's printed on their middles.
I often play with others
One of many instruments in an orchestra.
Some may think I am the best
The first violin maybe?
I usually feel tension
But I think that keeps me straight
The strings on a stringed instrument are under tension, which keeps them taut and straight.
I shout out loud and bold
And it's that shout that makes me great
The sound emitted by a musical instrument is what it's made for.
My master and I are best of friends
And our friendship keeps improving
A musician and his instrument can have a strong bond.
My friends and I unite as one
And that helps keep things moving
The sound of many instruments playing together in an orchestra can indeed be deeply moving.