A .45 ACP and a 9mm blast will likely sound very similar. That said, a suppressed .45 ACP will likely be quieter than a suppressed 9mm. But this is actually a very complicated question to answer because it depends on more than ammunition. It will also depend on the gun, barrel length, silencer being used, and more uncontrollable factors like the weather and humidity.
Technically, because standard .45 ACP ammo is subsonic, it is more effectively suppressed than 9mm ammo. 9mm is actually what can be called “supersonic”, which means that it breaks the sound barrier. Supersonic bullets are generally louder because a suppressor cannot quiet the sonic “crack,” so a suppressor will probably do more for a 45 ACP than a 9mm.
That being said, it really will depend on more factors than just ammo, and the difference we’re talking about here is small. In general, it’s best to use hearing protection when shooting a gun. But both a silenced .45 ACP and silenced 9mm will be below 140 dB, the OSHA threshold for hearing damage.