The information provided is very much limited. Is this was just one incident or she persistently complain about it. Secondly does she hear these voices frequently or just this one time. If these are persistent problem and she is not getting convinced by repeated explanation by you guys than , she may be suffering from psychosis
. At this age it may be secondary to some neurological cause so she need a neurological assessment and also psychiatric intervention.
Get her to doctors ASAP as it may progress also which can be difficult to manage.