I'd guess axles, aftermarket axles are atrocious, I can't exaggerate how bad they are. But first - Some important details are missing:
1. "pop" sounds pretty descriptive and I think we all get it - but honestly a clearer description would be good given the circumstances, the obvious has been touched on already with limited results.
Axles would be the most likely, particularly if they were previously replaced. But if you replaced them - usually even if you have symptoms they will at least be different, location, clicking instead of popping, blow apart, etc. No change makes blaming axles a little more amibiguous - but aftermarket axles are ***so*** bad, it can't be ruled out.
2. Any wrecks or damages?
3. Any major trans, differential, or suspension work in the past?
4. left, right, center or what? have someone help - ride in both seats and listen with windows up/down. Stand outside the car while slowly driving past/making the sound while standing on both sides of the car? A couple of minutes like that and it usually doesn't take long to get a hunch that it's emanating from the left/right/or center.
5. drive in a circle on dry pavement with the wheel turned to FULL lock, you will feel weird but just keep drifting in circles barely touching the gas. what happens?
6. shop can swap CV axles and see if the noise abates or "moves". they're the same on both sides and interchangeable.
7. aftermarket axles suck. you shouldn't have replaced both axles with the cheap garbage you now have in the vehicle. the original axles easily last 200,000 miles if greased/rebooted. i can almost guarantee those axles will not make 100,000 miles, they often have issues of fail in just a year or two. Simple google a combination of axle/aftermarket/subaru/failure/break/@I$U)(T*)(@ and you'll see zillions and zillions of examples. i have no idea how many blown apart on day one, clicking, vibrating, failed aftermarket Subaru axles i've seen over the years and I'm not even a shop/mechanic. it's crazy how bad they are.
I'd ask the shop if they still have your old axles and try to get them back. But i realize most people can't do that and they're probably already turned in for a core.
engine mounts could cause the CV's to fail. But they also don't want to replace random stuff guessing.
Not on Subaru's, unless maybe it's a lifted rig and you're rocking crawling it with locked differentials?
Good on your shop not wanting to guess - they would have been wrong.
If someone not familiar with Subarus needed confirmation it wasn't the mounts - that it does it only when warm does not sound like typical engine/trans mount/bushing symptom.