Larry explained it exactly right.

The first time I tried to put a condensor inside an accordion, it sounded awful and distorted, at first I thought it was the sound pressure levels, but it happened when playing soft too.

So I enlisted the help of one of the senior audio design engineers at Dolby Laboratoies, (that just happened to be my brother :)

We put the signal on a scope to see what was the issue. Every time the accordion was pushed or pulled without even playing a note it created a massive DC offset in the signal. This was because the mic diafragms were not vented.

Amps hate this. And speakers hate it even more.
It's a good way to actually blow some amps as well as speakers that do not have input protection.

DP