Your cabinet is a conversion from what appears to be a Neo-Geo, so there are no volume up/down buttons (those were Midway specific). Most of the Neo-Geos I've seen have a big test button mounted to the top of the coin vault. If you look straight in through the top coin door, it should be pretty conspicuous. It's some giant odd thing; it looks like it was probably intended to be used as a door sensor on an alarm system or something. This should bring up the test/operator menu, and you can adjust the volume from in there.
If you don't have that button, take a look at your JAMMA harness and see if there's any wires hooked up to the appropriate location (15 - parts side). If so, trace it and see where it goes. If not, just whack that pin with a wire to one of the many ground pins on JAMMA, and that'll bring up the menu; you might consider installing a button in that case, too.