First post, so I found this by accident but it appears to be repeatable.
1. Select a soldier to move the maximum possible distance he is able.
2. Before he reaches the end, hit C to make him crouch and it should make a constant walking sound.
At first I thought it caused a permanent walking sound, but if you un-crouch within the same turn it stops the sound (as the first time it happened, it went all the way through until the end of the battle).
After a little more testing it seems that you can move that same soldier in the following turns and it will stop the sound as well.
Another test also shows that it stacks, so you can have multiple soldiers move and then crouch and you will have the sound of multiple soldiers walking, until you move them again.
(it can be pretty overpowering when you have a 10 soldiers all walking at once )
Bug: Constant walking sound if you crouch while soldier is walking to a destination
Workaround: Move the soldier/un-crouch during any turn, or simply avoid it by crouching once you reach the destination.