Basically, any sound that's supposed to come from a PLUGIN is acting up. Whether it's the beeps from ELS when you flip on your lights, the CB clicks from GrammarPolice, the ALPR beeps from CalloutInterface, or the Panic button noise from STP – none of them are working. Oddly enough, all the other sounds in the game are fine, including the dispatch audio for calls. The ELS beeps don't even work when there are no plugins running, even after renaming / moving the mods folder.
This whole thing has been driving me nuts. I've combed through my .ini files a dozen times, made sure all the plugins have their sounds turned on with the right audio paths. Checked the audio settings in the game menu and my computer settings and drivers. No luck. I was hoping maybe someone else ran into a similar issue and knew a fix.
Oh, and this problem only crops up when I'm playing GTA V; all my other games are smooth sailing.