The bug

/stopsound has two success feedback messages:

• commands.stopsound.success: "Stopped sound <sound> for <player>"
• commands.stopsound.success.all: "Stopped all sounds for <player>"

/stopsound has an optional sound argument. When provided, it correctly stops the given sound, but gives the "stopped all sounds" message. When excluded, it correctly stops all sounds, but gives a broken "stopped sound for player" message with the sound parameter blank.

How to reproduce

1. /stopsound @a
→ It says "Stopped sound  for <you>"
Notice the two spaces between the two words, the sound argument is supposed to go here, but there isn't one because this is the wrong message.
2. /stopsound @a random.explode
→ It says "Stopped all sounds for <you>"
That's not right, it only stopped explosion sounds.