Firstly the speed of sound in airplane grade aluminum is "slightly" higher than in thin air, an order of magnitude would be my initial guess. So you are correct that any vibration or whatever from the engines will definately transferred to the whole fuselage. The actual "engine noise" is then another issue.

Secondly, it is not only the engine that you are hearing, also the "wind shear" against the whole fuselage creates audible sound...well, audible would be slight underestimate for the noise created in supersonic speeds.

When you drive on a highway with your car doing 100 km/h, barring you have V12 Ferrari, what are you actually hearing on a stable highway speed:
1. Noise from the tires
2. Noise of air rushing towards the windscreen

Then multiple this speed by 20 and imagine what happens to 2. So, whether you could hear the engine in normal highway speeds or not, the other elements create so much noise that it is not really relevant whether you could hear the engine noise or not as the intensity of the other sources of noise is significantly higher

But for the actual question, as the engines are travelling on a same pace as you, but the medium, where the sound would travel from the engines outside the fuselage to you sitting inside in the front, only allows speed of sound, i.e., less than your travelling speed. So, in principle you would not be able to hear the noise from the engine if, in some idealized world there would be only "free moving air" between you and the engines. edit: specified this sentence.
Howcome my message is #1, have I changed my settings somehow...