Ever heard a .22LR suppressed?
It's just like the "pffft" in the "movies".
Mix a .223 with subsonic ammo?
I've got a bit of trigger time with suppressors - ARs, MP5SD2, .22's. A suppressor works just like a muffler by delaying the exit of hot supersonic gases until they have had time to cool and slow. Naturally, if you choose a round with less powder, there are less combustible gases to cool and less noise.
132db - unsuppressed .22LR (as quiet as a suppressed .223)
91db - suppressed .22LR with top notch suppressor (41db reduction - still 6db over OSHA threshold of 85db)
68db - unsuppressed subsonic .22lr (CCI Quiet 2 - about as quiet as a loud conversation/car road noise)
Subsonic does nothing to make the suppressor quieter, it just reduces bullet flight noise (the tiny supersonic wave that accompanies the bullet). If you shoot subsonic out of a normal rifle, it will be quieter.
.223 subsonic ammo is just a bad idea. You've basically reduced yourself to a .22 Long and you are eventually going to have a baffle strike that trashes your suppressor.