Audio classification data balance
I think you might have confused the concept of training set, validation set, and actual input. The training set helps you build a model; the validation test let you validate that performance of your model. (sometimes people also use real life testing test to ensure the model is not overfitting) Once you have a model created, your model will be like a black box that takes your input (i.e., could be a single sample) and give you a classification result. So it really doesn't matter if your only 1% of your input data is a car sound; if the model is built properly it will be able to give you a good prediction.