Oh! I found this on site about Parkinson's disease.

snip

Sound and vision methods
When you freeze, decide which foot you are going to move first and then simply say 'One, two, three, step', 'ready, steady, go', or a similar phrase. Attempt to restart walking on the trigger word 'step' or 'go'. This can be done silently in your mind, spoken aloud by yourself or said by someone who is with you when you freeze. Whichever method you use, a clear commanding tone of voice will be the most effective to get you restarted.
Other strategies to try include using rhythm(again, either in your mind or aloud) by singing or humming a tune as you walk.

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Now that is interesting. Still looking for scientific explanation of why humming helps with balance.