Would someone sustaining a spell effect negate their Hidden status acquired by a successful Stealth roll?
I've checked the Essentials, and it mentions Keep Quiet under the requirements for Remaining Hidden on p.152/153
There is nothing inherent in the sustain rules that implies breaking the Keep Quiet requirement for remaining hidden.
In game, this implies to me that sustain is more concentration than it is chanting, stomping or otherwise taking un-stealthy actions.
Yes and No?
While I agree with Simon that there is nothing in the rules about it to dictate one way or the other, I think the GM needs to make a call on this as to how magic works in their particular world, and this needs to be spelled out EARLY on for the players.
If the GM dictates that spellcasting is something that requires constant chanting in order to sustain an effect, then I would expect that GM to rule that you could not gain Stealth because the character is making too much noise.
However, if the GM dictated that spellcasting did not require an oral component, but required only gestures, then it could be completely possible to sustain magic without making any noise.
Some GMs require neither vocal or oral components for spellcasting, which allows wizards and the like to keep casting even if they are silenced or bound in a game scenario.
I would say a chat with the GM about how magic casting works in their world is in order. If they haven't specified any house rules on this, then the base rules themselves definitely aren't in your way.