Factual error: When Christian Slater is talking on the phone in one scene, it is clear it is a cordless phone. When he slams it down, however, it makes that "ding" sound like older phones that really have a bell in them.
Factual error: Upon finding the cordless phone's base in the shed outside Mark's neighbor's house, the cop says, "There's the transmitter, which means the receiver could be in any house within 1,000 yards of here." I noticed no amplifier or anything to soup up the transmitter, and I know of no store-bought cordless phone with a range even remotely close to 1,000 yards. (01:09:05)
Answer: The first time I watched the movie, I kept thinking that Mark's parents were going to hear him and catch "Harry" in the act. When the girl was with Mark, Mark's parents pounded on the outside door; when Mark opened the door, his parents said they thought they heard him talking to someone. So, from outside the garage door, someone might be able to hear muffled voices but not the actual words. Why his parents cannot hear him in the basement when they are indoors lies in the props/scenery plus some inference. The numerous objects in the room (tapes, CDs, albums, guitar, drums, bongo drums, recording instruments, amps, etc.) indicate that Mark is into music - loud music - and electronics. He apparently was given garage/utility storage space to turn into essentially a studio for himself and a place to play his drums and music without disturbing his parents. The space has been sound-proofed - thick concrete walls, insulation, and cloth wall hangings to deaden the sound.
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