Problem with simple sound detection circuit using a microphone
I plotted the values you got in a spreadsheet:



You must admit that looks a bit like a sound wave! If you want to know if it is a soft sound or a loud sound you really need to see the maximum values (1023 in your case). If the maximum was 512 then clearly the incoming sound isn't as loud.

An alternative, if you just want loudness would be to filter the incoming signal, eg.







simulate this circuit &ndash; Schematic created using CircuitLab

This would smooth the incoming voltage so that consecutive readings would be in a narrower range.



Warning: Judging by the graph, you may be getting negative input into A0. The Arduino pins are not designed for negative voltage. You may want to use a diode to eliminate the negative component.