Cherry MX Blue vs Brown sound
Both switches have a bump. The difference is a small part called the click jacket. In a Blue, the jacket rides on the stem, catches on the way down, then snaps free with a click just as the key actuates, and resets with a softer click on the way up. A Brown has the bump without the jacket, so it only makes the two sounds every MX switch makes: the stem bottoming out in the housing, and the stem hitting the top of the housing when the spring pushes it back.
So a Blue adds two clicks to every keystroke and a Brown adds none. Pick Blue if you want every actuation confirmed out loud. Pick Brown if you want feedback you mostly feel, at a volume the people near you can live with.
Cherry MX Blue vs Red sound
This is the widest gap of the three. A Red has no jacket and no bump: a light 45 cN spring and a straight slide to the bottom. All you hear is a quick bottom-out and a quick return. A Blue needs 50 cN to actuate, peaks at 60 cN at the bump and clicks on the way through. Reds suit fast, light typing and gaming. Blues suit people who like to hear each key register.
Red switch vs brown switch sound
The closest pair. Neither clicks, both actuate at 45 cN and both travel 4 mm, so most of what you hear is the bottom-out, and that depends on the keyboard more than the switch. If you bottom out on every key, you'll hear little difference between them. The Brown's bump, 55 cN at its peak, can stop a light typist short of the bottom, which is quieter. In our recordings the Brown is the darker of the two, but they come from different boards and microphones, so part of that gap is the board.
What about MX Black and Gateron Red?
Two more linear switches are in Keyslam: the MX Black pack is a Leopold FC980M PD with Cherry MX Black switches, a linear with a heavier spring than the Red; the Gateron Red pack is a GMMK TKL with Gateron Red switches. Each is a different board and microphone again, so play them next to MX Red to hear how much the board changes a linear sound.
Which Cherry MX switch is quietest?
Of these three, Blue is the loudest because of its click. Red and Brown are close; how hard you bottom out and what the keyboard is made of matter more than the switch. Cherry also makes silent versions, such as MX Silent Red, with dampers built into the stem.
To quiet any board, try rubber O-rings under the keycaps, a desk mat and a lighter touch. If you want the sound without the noise, play a recording through headphones instead. And if all three are too much, the Laptop pack, a close-miked 2018 MacBook Pro, is a duller, subtler tap.
Why soft and slam sound different
Each pack is cut from one real typing recording. Every strike is detected and paired with its key-up, then the strikes are sorted by natural loudness into four bands and each band is leveled to a consistent target. Soft comes from the lighter presses, slam from the hardest ~8%. On a MacBook with Apple's motion sensor, Keyslam picks the band from how hard you actually hit; on other Macs you choose it. How force sensing works.

Cherry MX Brown sound test
Cherry MX Red sound test