The more details you give us, the better we can help you.

Do you use an external speaker system? I have been known to unintentionally hit the on/off button why adjusting the level.

I would also check to confirm you have sounds loaded in the Score Manager. Sometimes (but very rarely) things happen and you find they are not there. Then you need to Reassign Sounds.

Do you have sound in other programs?

Have you restarted the computer?

yes I have sound in other programs.

I have not restarted computer..it takes so long to reboot with EL Capitan, but i will as a last resort.

 I don't see anything about sound in Score Manager.

If you don't want to reboot (although I think it's better) you can try logging out of your account, and back in.