Quoted from LargemouthAss:I only have one pin (for now) but I cannot imagine being concerned about the quality of sound when I have multiple machines on at once...
You are correct. If you have multiple games going at once, better speakers are somewhat moot. It's when you are playing just one game at a time where it can make a big difference. Some games, especially modern ones, have incredible soundtracks that you can't fully appreciate with the stock speakers. I can still remember how amazed I was the first time I hooked up an external sub to my Tron. The start of light cycle multiball will shake you to your core! 
Hot Springs, AR
