I have bought an bluetooth heart rate monitor belt to use with my Android LG G3. There are many apps that can use that belt to record heart rate while I am running. Unfortunately none of them (I have tried Runtastic,Runkeeper,Endomondo,MapMyRun and so on) does a simple thing: play a sound when I cross a specific heart rate zone. This is a feature that a sport watch can do but not Android. Does exist an app with that feature?
It may not be exactly what you want, because it announces which zone you enter instead of just playing a sound, but SportsTracker does it. (Note that there are multiple apps with this name, so be sure to look for the icon.)
In fact there are other apps that have the functionality as well (e.g. Endomondo), but they will usually charge you a monthly fee for this feature. For SportsTracker it is the other way round. HRM support is free, but you will have to pay if you want to upload/export your track records.
I usually use Endomondo for keeping track records and SportsTracker for HRM announcements. It may be more complicated, but at least there is no need for a monthly subscription.