Quote: "I haven't placed any light markers"
Ok, your scene isn't lit with lights, so your ambience is most likely turned way down.
Try this, when doing your next test run, hit the ", and . " keys ( that's the comma and period keys, some say its the < & > keys)
Either way these two keys adjust in-game ambience up & down ( available light in an environment )
Try it, and when you get it the way you like hit the "Enter" key to lock it in
My games never have bugs. They just develop random features..
Lots and lots of random features...