I agree with everything Dave says. I use an internal mic on my Bb accordion (my zydeco box) and a clip on (audio-technica) on my C accordion. The clip on mic (which requires phantom power) is much brighter and louder than an internal mic. The internal mic also has the property of magnifying the sucking sound the accordion makes when you pull the bellows open.

You can fix the sucking sound of the internal mic by adding a foam windscreen around it. I have used a piece of nylon stocking, which also helps. I picked up that tip from someone on this board, but I don't remember who. It might have been Dana Mandell, who hasn't been around much lately.

You also play around with the direction that the mic is pointing inside the bellows. For reasons of minimizing feedback, it helps if the mic is pointing in towards your chest/stomach. If you do that, it also happens to not be pointed directly toward the air hole, so it helps with that problem at least a bit.

Thanks for the tip, David!