Have you ever fired subsonic ammo without a silencer? What is the db compared to nonSS ammo?
What advantage is there to subsonic ammo in non-silenced firearms?

Subsonic ammo doesn't suffer from destabilization that a supersonic bullet experiences when it slows from supersonic to subsonic in flight.

Subsonics have slightly less loud bang since the bullet is not breaking the sound barrier and making a sonic boom.

Subsonics are usually subsonic because a heavy bullet is used.  Most .45 ammo is subsonic.  

In regards to handgun ammo IMHO:
Subsonic= Cool catch phrase to market ammo.  
When using non-silenced firearms, there's really no inherent advantage to subsonic ammo.
You don't really notice any difference in loudness, a keen ear may notice the lack of the supersonic crack that goes with the bang.  
Also, considering that even the 124gr 9mm is still supersonic at 50yds, any destablization from the transonic transisition (if any) would be essentially irrelavant inside effective handgn range.

Subsonic vs supersonic in and of it self is largely meaninglesss in the unsilenced world.  The real question is heavy bullet at lower velocity vs lighter bullet at higher velocity, without any regard to whether the velocity is higher or lower than the speed of sound.

There is usually a recoil difference.

The subsonics tend to be not as snappy in recoil as the lighter, faster rounds of equivalent energy.  Therefore, subsonics tend to be "easier" to shoot accurately for most.

They also tend to penetrate deeper than their lighter, faster counterparts.  But as said, it's more of a factor of bullet weight rather than the speed of sound.

I would say that the only time you would notice a difference without a silencer would be a supersonic .22lr compared to a subsonic .22lr . In centerfire ammo the bang from the explosion of the powder is much louder than the supersonic crack. As someone said most .45 ACP ammo is subsonic and you can't shoot that without muffs or plugs.

Quoted: There is usually a recoil difference. The subsonics tend to be not as snappy in recoil as the lighter, faster rounds of equivalent energy.  Therefore, subsonics tend to be "easier" to shoot accurately for most. They also tend to penetrate deeper than their lighter, faster counterparts.  But as said, it's more of a factor of bullet weight rather than the speed of sound.

While we are discussing velocity, setting aside the speed of sound itself:
Additionally- generally speaking,  the higher velocity, the flatter trajectory.  (Although the heavier the bullet, the better it retains its velocity.  (Often at some point downrange, the lighter, faster bullet begins to drop more than the heavier, slower bullet.)

Not much difference at all in noise levels, that's detectable by the human ear anyway.  I shoot alot of suppressed .22 Remington Subsonics, and they are still pretty damn loud w/o the suppressor.
Short fat heavy bullets don't tend to upset too badly when they drop to subsonic speeds, due to their point of balance being about center.   You tend to notice destabilization in long pointed boat tails because more of their weight is centered closer to one end.
On the heavier/slower vs. Lighter faster, one can look at it this way.  Heavier bullets tend to retain velocity better, lighter bullets lose velocity faster.  Same principal that govens freight trains and passenger cars, heavier=more resistance to stop, Passenger car= less resistance to stop.  O.K. that's a shitty analogy, but it's all I can come up with at 1:48 AM