You don't actually have to cut speaker holes, or even make a volume knob (not if you have a coin door), since you can just reach inside and change the volume). Plenty of machines just had internal speakers with no visible grills, or with the speaker hole being in back or on the bottom (especially cocktails, caberets, and minis).
So really, just fasten them inside, and set the volume at an appropriate level. I have owned a metric buttload of arcade games in the last few years and I never once adjusted the volume on one of them more than once.
Coin doors can be had on ebay or off RGVAC for $20 all day long. Sometimes they need a fresh coat of paint, but that doesn't take long.
If you plan on building several machines then you can likely get a bunch of coin doors from a single person, thus cutting down on the shipping. Although if you live in a major city, then you should be able to find another collector who just has the things laying around everywhere (coin doors really build up when you get into games, everytime a game gets parted out it produces another extra coin door, which you don't need, since all your games already have them, I still have at least 3 spares kicking around, and that was after I sold most of them to other Mamers).