Changepoint Detection of a 2D signal
One approach would be to use the spectral flux of a sequence of STFT frames $X(n,k)$, namely

$$
\mathit{SF}(n) = \sum_{k = -N/2}^{N/2 - 1}{H(\left|X(n,k)\right| - \left|X(n-1,k)\right|)},
$$

where $n$ is the frame number, $N$ is the size (in samples) of each frame, and $H(x) = \frac{x + \left|x\right|}{2}$ is the half-wave rectifier function. Intuitively, it attempts to measure how much the spectrum changes from frame to frame.

The overall area of research is called onset detection. A good overview of techniques can be found in the following paper (which also describes spectral flux):


  S. Dixon. Onset detection revisited. In Proceedings of the 9th
  International Conference on Digital Audio Effects, volume 120, pages
  133-137, 2006.


An example implementation of spectral flux can be found in the NSGToolbox.