simulating directional interference to evaluate model performance
You do need to be careful how exactly you define the SNR. I think you are actually talking about SIR (signal to interference ratio). The way you describe it, it sounds like you plan to just compute the power (time averaged energy) emitted by the signal and interference and use that to get after the SIR. This would be fine but, as you mentioned, sometimes the interference is not present. I might think about using a different definition such as instantaneous SIR, since it sounds like sometimes you have signal + interference and sometimes it is just signal. For this reason, I'm not convinced that computing the power as you described is the right thing to do. 

I would suggest doing this: take the source audio file and interference audio file, figure out which is longer, zero pad the shorter one so they are of equal length. Now compute the metric, "sample-by-sample SIR", by doing $\frac{s[n]^2}{i[n]^2}$. Just be careful when $i[n]=0$. This is where knowledge of the noise (real noise, not interference) would be helpful so that you could ideally report SINR (signal to interference plus noise ratio) and avoid the denominator being $0$ problem.