Your best bet would be to forget about sound levels altering resistance and modify the schema to work directly with a DC level that is proportional to db. A cheap, readily available 50 cent mic, plus a 741 (or similar) operational amp a long with a handful of passives is all you really need to do the job. The task of the OP AMP is to amplify the very small AC signal from the mic. You'll also need a sample & hold circuit too. Do a google search on VOX projects. This may help point you in the right direction.
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