In the more recent versions they added a menu option to allow you to choose internal, external, or both speakers. The internal one is not switched off is it? Ignore me if you have already checked this.
No I kept checking that it was switched to "both" - plus tried it on internal as well. Silva have given me the key routine to reset the unit back to factory setting. Hopefully that might do it. If it resets the MMSI I might stick it on Ebay!