There could be a few reasons! Here are a few things to look out for:
Check your hardware. Make sure they’re completely plugged in, and make sure your hardware (the jack) is clean. If you’re using earbuds make sure they’re clean and not clogged up. For wired headphones, make sure there isn’t any damage along the wires of your headphones.
For wireless headphones, you could have interference from things like a metal desk in between devices. You should also ensure you aren’t too far from your device; this will weaken the connection and could affect your listening experience.