You're just not being ambitious enough. You have to make it fly high enough they can't hear it. Use a scramjet accelerated from the ground with a railgun and get the thing nearly into orbit, something like that.

Or shape the plane like a Busemann Biplane, so it doesn't produce a boom at all. I am a fan of parabolic-arc travel though. ICBT (inter-continental ballistic transport)

An interesting side effect of that is the impact on ICBM defense protocols. If everything looks like an inbound missle you couldn't protect against them anymore, unless your protocol becomes "shoot down any ICBx trip not registered" but now you have a theoretical gun pointed at every passenger transport, ready to fire automatically for the sake of missing paperwork.