deathcount doesn't detect players dead because of selectors @a @p @e @r

The way to reproduce this bug :
1. Set a deathcount variable : /scoreboard objectives add Deaths deathCount
2. Setup a hopper clock running on a command block with this command : /scoreboard players set @a[score_Deaths_min=1] Deaths 0
3. Detect the power from commandblock with redstone comparator
4. Put a command block next to the comparator : /playsound mob.wither.spawn @a

When a player dies, the sound is not played. However, the sound is played only when this one respawn.