I use "Round & Workout Timer+" from MedNotes Plus. It has a loud bell sound by default to indicate change of activity, but you can upload your own. It's also got lots of flexibility as to your "workout" and "rest" time periods, and you can save different patterns with their own name.

(I've got a 20min/10min one for tackling housework, and a 2min/1min for run/walking)