Run the speaker system for the cabinet to the audio jack.
That's the best way, and it's pure sound from the card.
HDMI sound is good, but really for a console style setup.
I have 3 MAME and 2 dedicated Arcade cabinets. All but one is set up this way. The one cabinet is all original, so I didn't mess with the set up. The rest all use 2.1 sound systems plugged into the audio jack of whatever I have running the cabinet.