There are three conditions in which a dispenser or dropper can play the 'Dispenser failed' sound:

1. A dispenser is activated while empty
2. A dropper is activated while empty
3. A fallible item dispenser behavior occurs and fails, such as flint and steel trying to dispense into a glass block

However, the effects of each of these is inconsistent. The first case plays the dispenser fail sound and emits a block_activate game event. The second case only plays the dispenser fail sound. The third case plays the dispenser fail sound and emits black smoke particles.

Given that these three cases all share the same sound effects otherwise, I believe that their not emitting a game event is a bug.