Probably a loose wire or broken solder point on the speaker, should be an easy fix if you can solder. Try calling Whites & they may fix it for you for free, they'll also tune it up & make it like new. Ive heard they'll do this even if the warranty is up, great customer service! Good luck!
Some models speakers wont work if you have anything plugged in to the headphone plug in. Even if you have an adaptor to take it from a 1/4" RCA to 1/8" RCA the speakers won't function correctly. Make sure nothing is plugged in before you send it in.
The speaker jacks on metal detectors break connection from the speaker when the headphone is plugged in, this is normal so you don,t get sound from both the speaker and the headphones. The jack may be worn out and it is not reconnecting the speaker. It should bean easy fix. As said try Whites or any electronics geek with a soldering iron can resolder a broken wire or replace a jack.