This must mean your interface is also mapped to - and = for your joystick's up and down. Check in MAME'S TAB menu under 'player 1 controls' and see. If it is, you will either need to re-map your keys for either up/down, remap MAME's volume controls to other keys, or re=program your interface with new key mappings. If these are all as they should be and both the the joystick mappings and the volume keys are mapped differently, it probably means your i-pac is buggered and sending 'ghost keys'.