Normally this happen to old laptop model with Windows 7 as OS, follow the steps below:
1. Go to Control Pannel, start - control panel
2. Then click on Hardware and Sound
3. After that click on Sound
4. Then you devices should come up, speakers, Digital Audio, and headphones
5. plug your headphones in
6. then select headphones
7. at the bottom it will say set as default
8. Select that and you are done