To add to the discussion,
Some of the older classics used an Ovular car type speaker. While the newer models were usually the 4 to 5" diameter round speakers.
Some classics such as Turbo, have 3 speakers. 2 up top, and I believe a 12" woofer in the base of the cabinet, which generally was for engine noises. And while many games may be mono... theres nothing like hearing the games that were in stereo, as stereo.
One could rig up a switch to toggle between different speaker outputs... Such as switching between a modern speaker set on the sides, and a cheap oval car speaker mounted in the center. Giving the best of both worlds, with a high quality jukebox sound, and old skool classic sound.
Another thing... I dont have a surround hooked up yet, but certain games that have 3 to 4 speakers are supposedly piped to the various channels in mame. This means games like TX-1 should have both front & rear speaker support. (can someone verify ?)