I am considering purchasing either Say Intentions or Beyond ATC. I know BATC now has its own traffic injector so I would hear ATC comms from other aircraft. However, I cannot find any information on whether SI plays comms from other local traffic, and if so, where it reads it from. Can anyone fill me in on this?
NB I am not interested in opinions on which has the “better” voices / features / cost etc.
One of the advantages of SI is that you’re able to hear comms from other pilots on the network. At this point it’s much more interactive than BATC. In reality though, the number of pilots in your vicinity on the SI network is often quite small depending on where you’re flying. You can always activate an option to hear communications from all over the world, or historical comms from pilots who have been in your area in the past.
But there is also the option of hearing AI generated comms. (No AI aircraft at this time, though. It’s apparently in the works.)
As @SamuraiSax wrote the “multiplayers” feature (it’s the name of the settings tab 
) can fill your earphone with chatters. These can be AI generated or replay from previous real other players flying in your current area. It’s cool to hear something that make sense in your zone and with voices that does’nt sound AI. The quality or non-quality (as-real-as-it-gets) is terrific.
Unfortunately, I’ve met several times (too many times), inconsistecies that kill my immersive experience.
First, replayed voices could be from very begginers. No offense. We have all started including me. But in the real world, it doesn’t match the minimum level required to make radio call 
You may argue that it matches also some unreal behavior of SayIntentions radio calls. I can’t stand anymore with the point in the altimeter settings and the density altitude in all ATIS messages.
Second, it’s very very disturbing to have messages with your own current callsign: 2 planes with the same reg at the same time in the same area. In my playfield around San Diego, it seems the real Cessna 172 N4975F is very popular on other SI users. It’s mine also That’s make your situation awareness totally scrambled.
And finally, some replayed sequences are not complete. You get only the call and not the answer/acknowledgment or the other way around. Don’t tell me it also happens in the real world, I know (1000 hours VFR pilot here). It doesn’t happened so often and so without any logic.
At the end of the day, does the SayIntentions chatter better than the payware ATC Chatters? Not sure for the area I’m flying (southern California).