Hi,
A few possibilities/things to check.
Firstly, have you accidentally turned the volume right down either on the cab remote or with the volume knob on the loco? (Sounds silly, but I've done this myself.)
When you programmed the loco, did you accidentally set the sound option to 'no sound'? You can check this on the remote after selecting the loco.
It is possible that there might be a spot of dust in the volume knob on the loco. Try turning it up and down a few times and see if that helps.
And finally, I have found that sometimes odd things can be cured by shutting down the loco, turning off the track power, powering off the Legacy base, and then starting everything back up again. (Sort of like a hard re-boot for a computer.)
Hopefully this will turn out to be one of the simple options. Otherwise, I'm sure that one of the more experienced experts here will be able to help.
Good luck.