Why is it that when we're listening to music it sounds much quieter over the roar of an active engine?

I believe the reason for this is the same way forces working against each other subtract. The decibels of an engine will cancel out part of the music's decibels that are coming from your speakers. The closer the sound is, the more decibels it can carry to your ear. If these decibels are being combated by another sound source, the original sound will be much quieter due to the cancellation of decibels.