Yeah, ammo can be pretty darned old and still function fine so long as it was stored ok.
Most 147gr 9x19mm standard pressure ammo is subsonic ANYHOW, so it may just work fine or it may, as has been said, be borderline for cycling if it's at the weaker end of the pressure range and you have a strong spring. (SoS=about 1120fps at sea level and about 1040fps at 1 mile high; muzzle vel of most 147gr standard pressure 9x19mm is 980-1050fps)
The most common purpose for subsonic ammo is use with a suppressor (silencer) since otherwise you get the sharp crack of the round breaking the sound barrier. It's going to be quieter than regular 9mm anyhow because of that. It's jacketed so it should be no prob in the Glock, other than, as has been said, cycling issues since it might be a little weak.