Been in touch with the technical staff at Mtronics they say the 12v is the nominal voltage and the units should be o/k
on 13.5v etc. The sound units have their own amp which when used at the top end voltage will cause the unit to get
quite warm, but it should not shut down on that voltage.
They said my friend could send it back and if found faulty would receive a new unit.
Can,t be fairer than that.
Thanks for the help again guys,