Expand getMixLength in AudioEngine

[diatrevolo]

Is your feature request related to a problem? Please describe.
getMixLength assumes the track that ends last starts at zero, effectively giving the duration of the longest track. This may not be the case if tracks start at different times, so this method would fail in some cases.

Describe the solution you'd like
getMixLength should account for tracks not starting at zero necessarily.

Describe alternatives you've considered
N/A

Additional context
N/A