Possible error sources

Unsorted notes about where errors may be introduced:

• Temporal smoothing: summing up several FFT windows will reduce time resolution. See also https://en.wikipedia.org/wiki/Short-time_Fourier_transform#Resolution_issues for a bit of background discussion.
• Does the app produce mono or stereo sound? Apparently, some devices can introduce arbitrary time offsets between the audio channels. Check if the android.hardware.audio.low_latency feature is used.