That does not look like dirt on the membrane to me, but deteriorated gold sputtering...
It eventually can partially lift the sputtering and reduce or vary conductivity and capacitance (noise).
Not sure if this is a good thing, or a bad thing... If you know what I mean. the sputters I see are thus spots where the conductive layer is gone? (From the backside).
Are these capsules re-membrane-able (is that a word?) And will it be worth the pennies?
Anyways, the pops we hear in the sample, are they caused by the membrane, or most likely not?
I know, must be hard to tell, but maybe... :-)
Thanks for the welcome, BockAudio!