I've had my MacBook Pro for half a year now. Lately, however, I have noticed that the volume has become much quieter, also when I plug in headphones (I must set to the highest level, so I hear anything!). What could that be? However, I do not think that the speakers are broken, because my laptop is actually nothing bad happened.
Try to vacuum clean with a vacuum cleaner maybe dust has come in or reset your MacBook
Did you test the headphones on another device?
Yes I have. I use my headphones almost every day on my phone and there are no problems.
Possibly. Can an SMC reset help, maybe a NVRAM reset.