Generating audio clip by superposing two files I'm currently trying to generate a set of audio files where some audio events (e.g. dog bark) are immersed in a background audio scene (e.g. crowd noise). 

I understand that if the event would have happened in the original scene, then there would also be some components due to the interaction between the sound and the environment. In other words, this should be, from my understanding, a convolutive mixture.

My question is, if I approximate the mixture by summing the two signals, thus doing an instantaneous mixture, am I losing a crucial part of the audio signal? In which circumstances is this a reasonable assumption, if any? 
My final objective is to train a classifier with the resulting audio.
We typically model audio to be a pretty linear thing, so yes, just add – you can multiply each input with a constant to weigh them differently.