I would change the tranny fluid every 40,000 mi. That's for the speakers. If both are out I doubt that it's a wiring problem. It wouldn't affect them both unless you turn them up real loud and blew the speakers. Odds are you got a problem in your amplifier system. If you can access the wires to the speakers just hook them up to any kind of cheap radio or something and see if they work then you'll know it's probably in the amplifier

Is it manual or CVT?

For the manual, changing the fluid is a moderately easy DIY project.  Scotty's 40,000 miles is a good interval, but it should specify the exact recommendation in the owners manual (Subaru likes to do 15 and 30k intervals, so it's probably 30 or 45k).

If it is CVT, it is definitely not DIY.  Subaru's early CVTs had a lot of problems, so they upped the warranty on them to 100K.  But the catch was that if you changed the fluid, that voided the warranty, so you wouldn't want to change it before 100k.  I don't recall what the warranty was on the 2015, but at 9 years old you're probably getting close to 100k anyway, in which case go ahead and change it.  If you have a good independent Asian car mechanic in your area, take it there, otherwise go to the dealer (but don't take it to Walmart or one of those quick-change oil places, it has to go to someone who knows Subaru CVT's).