As a young composer, I struggled to the point of nearly abandoning music writing altogether because of the fear that I had nothing to add. After some early inspirations and excitement writing orchestral music, I was suddenly overcome by the idea that everything had already been done in music. Why even bother?
Many of my peers seemed unconcerned or unaware of this existential question. I suspect though, that subconsciously they knew they needed to bring something new. I recall many students in composition class rushing to the avant garde techniques made popular by 20th century experimental composers. If you have a music degree or took a music appreciation course you'll likely know the list: flutter tonguing, using instruments in unconventional ways, odd time signatures or mixes of instruments (lute and trombone anyone?), prepared piano, tone rows etc. Why were these students reaching for avant garde techniques instead of just writing something beautiful for goodness' sake? (I'm not saying these techniques are illegitimate but that they are sometimes employed by composers as a short cut to making interesting music.) It's because writing tonal music doesn't feel new or worthwhile anymore, and even if you manage to write something interesting, you can always point to the "greats" who did it better.
It's extremely hard to write beautiful, tonal music that isn't derivative. It's much easier to hide in an atonal, through-composed screeching violin piece that cannot be objectively measured against any real standard than to write something as transparent and vulnerable as a simple piano piece with recognizable themes and a familiar, easy-to-follow form. So if it's hard to write music, and most of it seems to have been done already what choices are left to composers? You, the composer, are what's left. Your voice is what we need.
What does it mean to have your own voice as a composer? The real issue isn't "can you do something nobody has ever done before?" All of the influences, interests, abilities that you as a musician uniquely possess already give you the potential to have a distinct voice and add something of value to the world through your art. But that doesn't mean it will happen on its own. How then does one find this elusive voice?
Finding your voice means creating a style all your own. But that style can still exist within the borders of a well-defined genre. Nearly any band you can think of is part of a recognizable genre and yet they each have a unique musical sound. One key to finding your own is thinking about what you are already good at. What do you do well musically? Perhaps you are good at arpeggios on your instrument, or writing thoughtful, evocative lyrics, or maybe coming up with memorable or surprising melodies. In short, find your strengths and use them to your compositional advantage.
Another pointer to your musical voice is considering what you love in music. What are the frisson moments you have experienced listening to music? What has given you goosebumps or made you cry? Figure out what is happening musically in those moments and try employing those techniques in your own music. Of course, this can seem like a gimmick at first, but with practice you can learn how to utilize these tools in a compelling way that doesn't sound forced.
Finally, and possibly the most important way to find your voice, is to write an abundance of music. Stop trying to craft the one perfect piece and just write ten pieces this weekend. These will be sketches, not fully developed pieces. But what you learn from this process will be invaluable. After you write 50 or 100 pieces you will start to hear commonalities. You can tailor this though. It doesn't mean your stuck doing what you have always done. You will likely find tendencies that you don't like. Change them. Grow them. Listen to others and how they manipulate similar musical scenarios. You're not copying, you're learning by example. This is what all artists in every genre have done since the beginning of time. You look to the past and build on that foundation in your own way.
Put simply, your voice consists of your most deeply held musical opinions. Let those shine. One thing my peers in college had, the ones who used those strange techniques, is boldness. Be gutsy, then balance that with good taste. It may take you years to finally arrive at a voice you are happy with. You may change it again many times throughout your life, but never settle for merely copying the latest trends. Learn from them and make them your own.
I may not have it in me to create a new genre of music or to have a voice so distinct that it attracts millions of listeners. Still, I have something to say and so do you. By writing music with your own voice you're expressing yourself and your opinions about what is worthwhile, and someone else in the world will hear it and be glad you did.