Quite frankly, I don't care if it's a whistle/horn/megaphone or just a simple e-mail or phone call. If, as a hiring manager, you engage in a search assignment (internally or externally) and something significant changes on your end, you owe an explanation to the organization you represent as well as your search partner and the candidates (they have rights too). You-the-hiring-manager will look (really) bad if something has changed that could impact your hiring decision and you just bury your head is in the sand and don't tell anybody. It's just bad business, period. Collaboration is the key here.
Things happen - I get that. The game-changer could be budget, new management, or something more significant like a merger or acquisition that will directly affect the people involved in the hiring food chain. I'm not saying you need to share confidential information, but you do owe formal notification to all of the stakeholders. Tell them that you are delaying/making major changes to the search - sooner rather than later.
I've been on the receiving end of a couple of situations, and not knowing where things stand is no fun. Quite frankly, it's (totally) unprofessional - it makes the candidates nervous and sends the wrong message about your organization to the marketplace. While you may just be the messenger (router) through which the information flows, DON'T HOLD BACK.
Bad news can be good news if it's delivered early enough, but late-in-the-game bad news is always negative. That's my two cents...