I have multiple VSTs, both using Play and non-Play libraries. But Hollywood Orchestra becomes really faint and hard to hear: the volume drops significantly and it becomes muted as soon as I start playing a song in Cakewalk Sonar Platinum. As soon as I load the instruments, they sound punchy and full, but when I start playing the song, many of the Hollywood Orchestra patches suddenly become very muted (and if I try pounding the midi keyboard keys, it still sounds muted and quiet). If I reload the patch, it becomes punchy and loud again, only to become muted when I play the song in the DAW.
 
 
Symphonic Orchestra doesn't give me that problem, nor do the non-Play instruments. The only patches that I've found to do that are Hollywood Orchestra patches.
 
I have a Windows 10 PC using Sonar Platinum and 32GB of RAM. Any idea what's causing Hollywood Orchestra song to become so muted?

Check the mod wheel plus any CC automation you might be using. There used to be a 'Zero Controllers When Play Stops' option in in Prefs / Midi. If it's still there uncheck it.