MacBook Pro no longer recognises speakers

I have a 2009 MBP with Yosemite and Bose speakers purchased from Apple. These will work fine when plugged into my iMac with Yosemite. However, they don't work when plugged into the MBP. They have worked in the past on the MBP, but not now. The MBP internal speakers work fine. Is this the failure of an internal component in the MBP or is there a setting that I can access to make them work?

Try this it may help identify the problem.
1. Plug the speakers into the iMac and make sure they are working.
2. Open the Sound preference pane and check the settings in the Output tab.
3. Connect the speakers to the MacBook Pro and make sure the settings are the same as on the other computer. Does the sound work now?

Have you added any software to the MB Pro that isn't on the iMac? Particularly sound-related extensions such as Boom.

Many thanks Sly dude. I am now back from my New Year in London and have done what you suggested and it worked perfectly.
A Happy NewYear to you.

Glad that sorted things out for you. It was the easiest way I could think of to sort out whether a software swirling was causing the problem.