I've heard this many times, and upon researching it, I too was disappointed when I discovered that Simon does not mean "shifting sand", however the teaching is true when you discover what his name means is based on his earthly father's name, Jonah. Simon bar Jonah (Simon son of Jonah). The name Jonah has etymological roots that can indeed mean mud or wet sand or shifting sand. If you do a casual search, you'll find Jonah means "dove", but search on etymological roots of Jonah's Hebrew name meaning and you'll find such as the following:
"First there is root (ywn). It yields the masculine noun (ywn), meaning mire, which is wet or soft mud (Psalm 40:3, 69:3)."
So it could be implied that Peter's original name meant, "He who hears" but is founded on "shifting sand" in the flesh based on his sin nature inherited from his earthly father, however, now that He has made His profession of faith, He is now founded on The Rock, His Heavenly Father and His Lord Jesus Christ, and as such is now a rock himself, not merely someone who casually hears, but effectually does.
So, if a little digging is done, his name does really imply someone who hears but is like a reed that is just blowing in the wind or has no firm foundation because he's here and there.
Hope this helps.