I first picked up a guitar when I was about 7. Ever since I've been playing it, and I even performed a solo at a concert (Little Rover, by the stupendium!) I have 3 guitars, all acoustic. One hasn't been used in about a year and I'm thinking of selling it (any takers?) another is currently sidelined because of a broken string (not from my earlier question, that one has already been fixed) so I only have one that is in good enough shape to be played. The actual question here is for my guitar, it is currently strung with the top 3 strings brass and lower 3 nylon. I don't play professionally (yet) but I'm preparing a music festival in my town, and I want to know the advantages of both. Which one would produce a louder sound?
Neither steel nor nylon strings on their own produce very much sound at all, even when stretched and plucked. A vibrating string of any material just doesn't couple with enough air for the sound of it to be clearly audible.
That is why guitars have large, hollow, wooden bodies. The body of a guitar couples the energy of the vibrating string to a larger amount of air, acting as an amplifier for the string. The things that make a guitar sound louder or quieter are the construction of the bridge, top, back, and sides. There are also resonator guitars that have special designs intended to get more volume out of a guitar without making it much larger.
So the answer to which string is louder is neither. They are both extremely quiet on their own.
If you are curious about which kind of guitar is louder, then guitars that can be strung with steel strings are capable of more volume than guitars that must be strung with nylon strings.
In terms of "advantages" and "disadvantages" of nylon versus steel strings, it doesn't really work like that. You (almost) never have a choice between whether to put nylon or steel strings on the same guitar. Nylon string guitars must use nylon strings; steel string guitars must use steel strings.
On a nylon string guitar:
- The advantage of nylon strings is you can put them on the guitar and the guitar will sound and play correctly.
- The disadvantage of steel strings is if you try to put them on, it will be difficult and you run the risk of damaging the guitar.
On a steel string guitar:
- The advantage of steel strings is they will fit and work as intended and the guitar will sound and play correctly.
- The disadvantage of nylon strings is they will be very hard to put on and the guitar won't sound or play correctly at all. At least in this case there is no chance of damage to the guitar.
The answer to your question is
Yes, you should keep it like that else you risk damaging your guitar. As others stated, the three brass strings are nylon cores wound in brass (or other metal). If you buy a pack of "nylon" strings you will likely find the same configuration. It is standard.
On the off chance that someone put nylon on an acoustic guitar, then you would be fine to try out metal acoustic guitar strings. I HIGHLY doubt that this is the case, it sounds like your guitar is a "classical" guitar.
There is a huge difference in Young's modulus between nylon and steel, see: What is "Young's modulus", and how does it relate to guitar?
Nylon strings change pitch less than steel when bent, either voluntarily or involuntarily, so they provide more stable intonation.
Nylon strings sound inherently more mellow, as they produce less high harmonics.
That being said, it's not really a choice whether to put steel or nylon strings on a guitar. As the others said, you need an appropriately made instrument to benefit from the string qualities. Moreover, putting steel strings on a classical guitar may damage the instrument due to increased tension.