This one is an easy change, and one I alluded to earlier.
Hush and Mage Stealth have their own special effect (+3 to Stealth, -5 to be heard) but it's just as easy to define the spell in terms of Silence, the advantage.
Regular, Resisted by Will.
Subject cannot make any sound, accidentally or on purpose. For purposes of stealth or being heard, this counts as 3 levels of Silence (p. B85).
Everything is as the spell Hush in GURPS Magic.
Subject gains 1 level of Silence per energy point in the spell.
Cost: 1 per level of Silence, half cost to maintain (round up.)
Time to Cast: 1 second per energy point.
I think that makes the spells more broadly useful and mechanically consistent.