Zerg: How to regularly create drones without watching?

There is no audio cue for a spawned larva.

However, there are several ways to work around this:

1. Larvae spawn every 11 seconds (except in co-op), which you will naturally get a feel for as you spend more time playing Zerg.
2. The icon for selecting the Larvae attached to the selected Hatchery has the number of selectable Larvae displayed in the lower-right corner, which you can check after pressing 4 with a quick glance.

As Nzall states, there's no audio cue, and as you mention, the sounds for successfully or unsuccessfully creating a drone are the same.

However, what you can notice is whether or not your minerals decrease when you hit the hotkeys. As you get used to the timings of larva spawning, noticing this out of the corner of your eye should be enough to tell you on the (hopefully rare) occasions when you miss the timing.

Your problem is due to the fact you are not organizing your game properly.

Macro cycle should be like this:

1. make sure you have one queen next to each hatchery
2. go over each hatch(default key is backspace, many change it to space) and inject.
3. build enough overlords so you are not supply blocked when larva pops

(do other stuff)

3. when larva pops build units or drones

tl;dr - you do not build units at random points in time and wonder if there is larvae or not, you do it in cycles.