I have a custom 300/221 Fireball (same as a 300 Whisper, but I can't call it that because that moniker has been copyrighted/owned by JD Jones). There is a whole line of these cartridges that were developed as dual purpose rounds. They perform well in subsonic mode for CQB, but can also be used in hunting situations (usually supersonic). To get a better look at these cartridges go to:
300 Whisper ® | SSK Industries
Another great site for info on the 300 Whisper, or subsonic rounds, suppressors, and other related info is:
300 Whisper Page
The big problem with subsonic rounds and hunting has already been touched upon by eddybo: bullet expansion. Most of the subsonic rounds based on the Whisper case utilize rifle bullets. These bullets usually need at least 1,700 to 1,800 fps in order to expand. This is way above the speed for breaking the sound barrier. At slower speeds (including subsonic), the usual rifle bullets simply will not expand - they just pencil through. That is why these cartridges tend to use heavy-for-caliber bullets. They begin to destabilize and tumble once they hit the target. I use the 240 gr. SMK in my 300. It takes a 1-in-8 twist to stabilize them and they do upset and tumble easily much below 950 fps.
Lately, some custom bullet-makers have developed specialized bullets for the Whispers that open up and have great terminal performance at subsonic velocities. Here are a couple:
Home - Outlaw State Bullets LLC
They ain't cheap, but boy do they work.
Corbin Mfg. has now legitimized the 300 Blackout (essentially the 300 Whisper). Here is a link showing subsonic performance of a bullet made by the Bullet Depot:
Subsonic Bullet Design
The 450 Beowolf and the 458 SOCOM tend to have better bullet performance at subsonic speeds because they utilize pistol bullets which are designed to open up at lower velocities. Their Achilles heel is that they have a low BC, so the drop beyond 50 yds. is very dramatic. Additionally, I believe they must utilize the AR-10 platform in a semi-auto platform (but I could be mistaken).
Something else to keep in mind is that the speed at which the sound barrier is broken varies by elevation, and sometimes other weather conditions. A bullet that is subsonic at sea level may be supersonic at the 7000 ft level. Generally, subsonic speeds occur somewhere between 1,040 and 1,080 fps. But again, this depends on you elevation.
To answer the other question about noise and the sound barrier: no - it is no more quiet whether the bullet is traveling 50 fps or 500 fps over the sound barrier. It is the same "crack" of a sonic boom. But what is interesting, is when the bullet is "trans-sonic". Most of us think of breaking the sound barrier as being over a specific speed. The fact is that it is a narrow range of fps. In this "trans-sonic" window we hear something more than the usual silenced round, but it is not a super-sonic "crack" either.
You can go to youtube and watch videos of jets breaking the sound barrier. Just watch for the "bloom" of the sonic shock wave as it begins to develop behind the tail of the aircraft. This trans-sonic window is also evident in longrange shooting. We have drop tables for our bullets. But once velocity of our bullet drops into the "trans-sonic" window, it is hard to get reliable drop info on a repeatable basis. Too much is happening, plus the bullet itself is probably beginning to destabilize.
Probably too much info here, but welcome to the world of subsonic shooting.