Why do you think it is necessary to convert the ratio, in the range of 0 to 1023, of the circuit voltage to the reference voltage, to an "absolute" voltage, in order to determine when the sound starts or ends?
The returned value is above a threshold, or it isn't. Quit wasting time converting the value to a different range, so you can use a different threshold.
Learntoseparatevaluesyouareprintingwithsomewhitespace,sotheyarereadable.
And, anonymous printing sucks. Identify what you are printing.
What you are timing now is how long it takes loop() to execute, which has NOTHING to do with how long the analog pin is above the threshold.