Identify Where Singing Starts in a Voice Only Recording
If the background noise is white-ish, you could measure spectral flatness and consider it to be voice when the amplitude is above some threshold and the spectral flatness is below some threshold.

Basically you just take an FFT of a chunk of the signal, then divide the geometric mean of the magnitude of the spectrum by the arithmetic mean.

You could also use a band-pass filter to emphasize only the frequency regions where the human voice usually sits (as simple as setting the unwanted regions of the FFT to 0 before measuring the spectral flatness)