There are two ways I see that could work, assuming that you need to build the tunnel right where the ground is scanned:
Cancelling the tunnel's effect on active sonars is possible by using technology similar to that in today's noise cancelling headphones. The tunnel could in that way cloak itself by sending out "anti-sound" that cancels any reflection or shadow.
I imagine one could record the incoming waves and use some kind of transform (similar to the Fourier transform) to figure out from which directions the waves come, and then send out the sound required to cancel any reflections and create new sonic waves in place of the shadowed waves. Even though this would technically be possible with today's technology and would not be mathematically that advanced, this would require acoustic sensors and actuators (i.e., microphones and speakers for recording sending out sound waves in the ground) all along and all around the tunnel.
Passive acoustic coupling to make the tunnel transparent to sonic waves
A couple of years ago, Steve Mould releases a YouTube video in which he explained why some interfaces reflect sound waves and why some interfaces don't reflect sound waves. He gave ultrasound gel as an example, which is used during ultrasound scans as an acoustic coupler to prevent sound waves from reflecting back once it hits the skin. Basically, a sonic wave will be (partially) reflected when it encounters an interface between two mediums with different acoustic impedance.
This is analogous to when a light wave hits an interface between two objects with different refractive index, such as the interface between air and water or between air and glass. A medium that transmits light and has the same refractive index as air will be invisible in air, and a medium that has the same refractive index as water will be invisible when submerged into water (note that the reason for example glass is not completely invisible is because it has a different refractive index than air and will therefore both bend light as well as reflect some of the light that hits it).
The same goes for sonic waves—an interface between two medium with the same acoustic impedance will not reflect any sonic waves. Now, depending on how the tunnel is constructed, there may be several parts of a tunnel which all have different impedance. If the tunnel has some kinds of walls to prevent earth from falling in, those may have one impedance. The air inside of the tunnel have another impedance, probably much, much lower than that of the ground (for example, the human body has about 3,000 the impedance of air according to the video and it's probably not to dissimilar with the ground relative to air). All of those interfaces might reflect sonic waves. There are however several ways to tackle this.
Perhaps a bit unintuitively, the sonic waves will be reflected less the more interfaces they have to pass, assuming that that allows the relative change in impedance across each interface to be lower. The ideal would be to have an impedance gradient between the ground and the air inside the tunnel, but several interfaces that helped transition the impedance in several small steps would probably be good enough. This would allow sonic waves to propagate into the air inside of the tunnel without being reflected. However, the sound waves would will still travel slower inside of the tunnel than outside of it, making them become out of phase when they exit the tunnel again compared to the waves that passed next to the tunnel and never were slowed down, giving rise to diffraction patterns with constructive and destructive interference which could give the tunnel away.
Another strategy would be to add some kind of structure inside of the tunnel that could transmit the sonic waves straight through the tunnel with the same speed as waves in the ground outside of the tunnel, effectively making the sonic waves propagate through that structure instead of through the air inside the tunnel. For example, metal rods between the tunnel walls going in all possible directions could be added, or perhaps some sort of thredimensional metal grid. This would of course make it difficult to walk inside the tunnel, but perhaps it would be possible to create the structure in such a way that the waves propagated around the innermost part of the tunnel, freeing that for walking or crawling. This type of structure would of course make the tunnel take up a significantly larger volume, and it's not really clear to me exactly how you would design the structure to make the waves propagate with the correct speed in all directions, but maybe there is some clever way to design it that would make the tunnel as good as transparent to sonic waves.
Clearly, this is a rather advanced concept (both mathematically and technically, I would guess). But maybe the team of robbers can pull it of if they know a mathematician who can come up with a theory that describes how a tunnel can be constructed that is coupled acoustically to the ground in an effective way, and an engineer who can help them build such a tunnel.