The low pressure means not much happens. Further low pressure makes a dramatic difference in energy needs.

The maximum designed speed is less than the speed of sound inside the tube, so I don't believe this will be a problem.

Huh, I thought that speed of sound increases when pressure decreases, but it seems to be the opposite.

Yep. Intuitively, sound propagates when particles bump into each other and transfer energy. Lower pressure means lower density, so there are fewer particles, which means it takes longer before an energized particle happens to bump into another one and propagate the wave.

Here's a graph I found. It appears highly non-linear.

https://sooeet.com/speed/speed-of-sound-in-atmosphere-graph....

Probably because that isn't just v vs. pressure, it probably takes the temperature into account as well.

The speed of sound is invariant to changes in pressure, it is temperature that matters.