Some safes can be opened by sound, much as in the movies, but most that were manufactured, after the mid 60's are not susceptible to the method mentioned here.

A high gain amplifier is used.  A small, direct-coupled amplifier can be purchased for about 5 dollars from the various electronic supply houses.  A contact microphone (i.e. one that "hears" vibrations rather than actual sound) should be employed.  BE SURE to use a matching transformer if the impedances do not match' and in a pinch, a crystal phone cartridge can be employed as a contact microphone.

How to Open a Safe by the "Sound" Method will acquaint you with the use of sound in determining the combination to open safes.