Speaker test
Test each side on its own, then together. If "left" comes out of the right side, your speakers are plugged in backwards — it happens more than you'd think.
Turn your volume down before you start, especially with headphones on. Tones are louder than they look.
Low (bass)High (treble)
Pick a side to start. You should hear a steady tone from that side only.
Hearing nothing at all? Check the volume and the output device first — Windows loves quietly switching to a monitor's speakers that don't exist. One side dead, crackling, or rattling on the low tones? That's a repair — bring it in.